Add form_with to unify form_tag/form_for. (#26976)

* Add form_with to unify form_tag/form_for.

`form_tag` and `form_for` serve very similar use cases. This
PR unifies that usage such that `form_with` can output just
the opening form tag akin to `form_tag` and can just work with
a url, for instance.

`form_with` by default doesn't attach class or id to the form —
removing them on fields is moved out to a default revisiting PR later.

Ported over old tests where applicable to ensure maximum coverage,
but left some commented out because they don't yet apply (e.g.
`fields_for` later being replaced by `fields`).

[ Kasper Timm Hansen & Marek Kirejczyk ]

* Add fields DSL method.

Strips `_for` and requires models passed as a keyword argument.

* Document form_with.

Graft the `form_for` docs: rewrite, revise and expand where
needed.

Also test that a `format` isn't used when an explicit URL
is passed.

* Enable remote by default.

Brand new world! Forms submit via XHRs by default, woah.

* Invert `include_id` to `skip_id`.

`skip_id: true` reads better than `include_id: false` (since the
`include_id` default is true).

* Invert `remote` to `local`.

Since forms are remote by default, the option name makes more sense
as `local: true`.

* Invert `enforce_utf8` to `skip_enforcing_utf8`.

* Refer to the brand spanking new rails-ujs.

Soon to be bundled in Rails proper, so jquery-ujs is out.

* Make `form_with` a bit more composed.

The flow is still not quite what it should be because the legacy
methods and these new ones pull at opposite ends.

Lots of options have been renamed, so now the new pieces don't fit
in so well.

I'll try to work on this in later commits after this PR (it's likely
there's a much better way to structure this whole part of Action View).

2 files changed, 2475 insertions, 2 deletions

diff --git a/actionview/lib/action_view/helpers/form_helper.rb b/actionview/lib/action_view/helpers/form_helper.rb
index 9bffe860db..1e1ddb4d71 100644
--- a/actionview/lib/action_view/helpers/form_helper.rb
+++ b/actionview/lib/action_view/helpers/form_helper.rb
@@ -474,6 +474,242 @@ module ActionView
       end
       private :apply_form_for_options!
 
+      # Creates a form tag based on mixing URLs, scopes, or models.
+      #
+      #   # Using just a URL:
+      #   <%= form_with url: posts_path do |form| %>
+      #     <%= form.text_field :title %>
+      #   <% end %>
+      #   # =>
+      #   <form action="/posts" method="post" data-remote="true">
+      #     <input type="text" name="title">
+      #   </form>
+      #
+      #   # Adding a scope prefixes the input field names:
+      #   <%= form_with scope: :post, url: posts_path do |form| %>
+      #     <%= form.text_field :title %>
+      #   <% end %>
+      #   # =>
+      #   <form action="/posts" method="post" data-remote="true">
+      #     <input type="text" name="post[title]">
+      #   </form>
+      #
+      #   # Using a model infers both the URL and scope:
+      #   <%= form_with model: Post.new do |form| %>
+      #     <%= form.text_field :title %>
+      #   <% end %>
+      #   # =>
+      #   <form action="/posts" method="post" data-remote="true">
+      #     <input type="text" name="post[title]">
+      #   </form>
+      #
+      #   # An existing model makes an update form and fills out field values:
+      #   <%= form_with model: Post.first do |form| %>
+      #     <%= form.text_field :title %>
+      #   <% end %>
+      #   # =>
+      #   <form action="/posts/1" method="post" data-remote="true">
+      #     <input type="hidden" name="_method" value="patch">
+      #     <input type="text" name="post[title]" value="<the title of the post>">
+      #   </form>
+      #
+      # The parameters in the forms are accessible in controllers according to
+      # their name nesting. So inputs named +title+ and <tt>post[title]</tt> are
+      # accessible as <tt>params[:title]</tt> and <tt>params[:post][:title]</tt>
+      # respectively.
+      #
+      # By default +form_with+ attaches the <tt>data-remote</tt> attribute
+      # submitting the form via an XMLHTTPRequest in the background if an
+      # an Unobtrusive JavaScript driver, like rails-ujs, is used. See the
+      # <tt>:remote</tt> option for more.
+      #
+      # For ease of comparison the examples above left out the submit button,
+      # as well as the auto generated hidden fields that enable UTF-8 support
+      # and adds an authenticity token needed for cross site request forgery
+      # protection.
+      #
+      # ==== +form_with+ options
+      #
+      # * <tt>:url</tt> - The URL the form submits to. Akin to values passed to
+      #   +url_for+ or +link_to+. For example, you may use a named route
+      #   directly. When a <tt>:scope</tt> is passed without a <tt>:url</tt> the
+      #   form just submits to the current URL.
+      # * <tt>:method</tt> - The method to use when submitting the form, usually
+      #   either "get" or "post". If "patch", "put", "delete", or another verb
+      #   is used, a hidden input named <tt>_method</tt> is added to
+      #   simulate the verb over post.
+      # * <tt>:format</tt> - The format of the route the form submits to.
+      #   Useful when submitting to another resource type, like <tt>:json</tt>.
+      #   Skipped if a <tt>:url</tt> is passed.
+      # * <tt>:scope</tt> - The scope to prefix input field names with and
+      #   thereby how the submitted parameters are grouped in controllers.
+      # * <tt>:model</tt> - A model object to infer the <tt>:url</tt> and
+      #   <tt>:scope</tt> by, plus fill out input field values.
+      #   So if a +title+ attribute is set to "Ahoy!" then a +title+ input
+      #   field's value would be "Ahoy!".
+      #   If the model is a new record a create form is generated, if an
+      #   existing record, however, an update form is generated.
+      #   Pass <tt>:scope</tt> or <tt>:url</tt> to override the defaults.
+      #   E.g. turn <tt>params[:post]</tt> into <tt>params[:article]</tt>.
+      # * <tt>:authenticity_token</tt> - Authenticity token to use in the form.
+      #   Override with a custom authenticity token or pass <tt>false</tt> to
+      #   skip the authenticity token field altogether.
+      #   Useful when submitting to an external resource like a payment gateway
+      #   that might limit the valid fields.
+      #   Remote forms may omit the embedded authenticity token by setting
+      #   <tt>config.action_view.embed_authenticity_token_in_remote_forms = false</tt>.
+      #   This is helpful when fragment-caching the form. Remote forms
+      #   get the authenticity token from the <tt>meta</tt> tag, so embedding is
+      #   unnecessary unless you support browsers without JavaScript.
+      # * <tt>:local</tt> - By default form submits are remote and unobstrusive XHRs.
+      #   Disable remote submits with <tt>local: true</tt>.
+      # * <tt>:skip_enforcing_utf8</tt> - By default a hidden field named +utf8+
+      #   is output to enforce UTF-8 submits. Set to true to skip the field.
+      # * <tt>:builder</tt> - Override the object used to build the form.
+      # * <tt>:id</tt> - Optional HTML id attribute.
+      # * <tt>:class</tt> - Optional HTML class attribute.
+      # * <tt>:data</tt> - Optional HTML data attributes.
+      # * <tt>:html</tt> - Other optional HTML attributes for the form tag.
+      #
+      # === Examples
+      #
+      # When not passing a block, +form_with+ just generates an opening form tag.
+      #
+      #   <%= form_with(model: @post, url: super_posts_path) %>
+      #   <%= form_with(model: @post, scope: :article) %>
+      #   <%= form_with(model: @post, format: :json) %>
+      #   <%= form_with(model: @post, authenticity_token: false) %> # Disables the token.
+      #
+      # For namespaced routes, like +admin_post_url+:
+      #
+      #   <%= form_with(model: [ :admin, @post ]) do |form| %>
+      #     ...
+      #   <% end %>
+      #
+      # If your resource has associations defined, for example, you want to add comments
+      # to the document given that the routes are set correctly:
+      #
+      #   <%= form_with(model: [ @document, Comment.new ]) do |form| %>
+      #     ...
+      #   <% end %>
+      #
+      # Where <tt>@document = Document.find(params[:id])</tt>.
+      #
+      # === Mixing with other form helpers
+      #
+      # While +form_with+ uses a FormBuilder object it's possible to mix and
+      # match the stand-alone FormHelper methods and methods
+      # from FormTagHelper:
+      #
+      #   <%= form_with scope: :person do |form| %>
+      #     <%= form.text_field :first_name %>
+      #     <%= form.text_field :last_name %>
+      #
+      #     <%= text_area :person, :biography %>
+      #     <%= check_box_tag "person[admin]", "1", @person.company.admin? %>
+      #
+      #     <%= form.submit %>
+      #   <% end %>
+      #
+      # Same goes for the methods in FormOptionHelper and DateHelper designed
+      # to work with an object as a base, like
+      # FormOptionHelper#collection_select and DateHelper#datetime_select.
+      #
+      # === Setting the method
+      #
+      # You can force the form to use the full array of HTTP verbs by setting
+      #
+      #    method: (:get|:post|:patch|:put|:delete)
+      #
+      # in the options hash. If the verb is not GET or POST, which are natively
+      # supported by HTML forms, the form will be set to POST and a hidden input
+      # called _method will carry the intended verb for the server to interpret.
+      #
+      # === Setting HTML options
+      #
+      # You can set data attributes directly in a data hash, but HTML options
+      # besides id and class must be wrapped in an HTML key:
+      #
+      #   <%= form_with(model: @post, data: { behavior: "autosave" }, html: { name: "go" }) do |form| %>
+      #     ...
+      #   <% end %>
+      #
+      # generates
+      #
+      #   <form action="/posts/123" method="post" data-behavior="autosave" name="go">
+      #     <input name="_method" type="hidden" value="patch" />
+      #     ...
+      #   </form>
+      #
+      # === Removing hidden model id's
+      #
+      # The +form_with+ method automatically includes the model id as a hidden field in the form.
+      # This is used to maintain the correlation between the form data and its associated model.
+      # Some ORM systems do not use IDs on nested models so in this case you want to be able
+      # to disable the hidden id.
+      #
+      # In the following example the Post model has many Comments stored within it in a NoSQL database,
+      # thus there is no primary key for comments.
+      #
+      #   <%= form_with(model: @post) do |form| %>
+      #     <%= form.fields(:comments, skip_id: true) do |fields| %>
+      #       ...
+      #     <% end %>
+      #   <% end %>
+      #
+      # === Customized form builders
+      #
+      # You can also build forms using a customized FormBuilder class. Subclass
+      # FormBuilder and override or define some more helpers, then use your
+      # custom builder. For example, let's say you made a helper to
+      # automatically add labels to form inputs.
+      #
+      #   <%= form_with model: @person, url: { action: "create" }, builder: LabellingFormBuilder do |form| %>
+      #     <%= form.text_field :first_name %>
+      #     <%= form.text_field :last_name %>
+      #     <%= form.text_area :biography %>
+      #     <%= form.check_box :admin %>
+      #     <%= form.submit %>
+      #   <% end %>
+      #
+      # In this case, if you use:
+      #
+      #   <%= render form %>
+      #
+      # The rendered template is <tt>people/_labelling_form</tt> and the local
+      # variable referencing the form builder is called
+      # <tt>labelling_form</tt>.
+      #
+      # The custom FormBuilder class is automatically merged with the options
+      # of a nested +fields+ call, unless it's explicitly set.
+      #
+      # In many cases you will want to wrap the above in another helper, so you
+      # could do something like the following:
+      #
+      #   def labelled_form_with(**options, &block)
+      #     form_with(**options.merge(builder: LabellingFormBuilder), &block)
+      #   end
+      def form_with(model: nil, scope: nil, url: nil, format: nil, **options)
+        if model
+          url ||= polymorphic_path(model, format: format)
+
+          model   = model.last if model.is_a?(Array)
+          scope ||= model_name_from_record_or_class(model).param_key
+        end
+
+        if block_given?
+          builder = instantiate_builder(scope, model, options)
+          output  = capture(builder, &Proc.new)
+          options[:multipart] ||= builder.multipart?
+
+          html_options = html_options_for_form_with(url, model, options)
+          form_tag_with_body(html_options, output)
+        else
+          html_options = html_options_for_form_with(url, model, options)
+          form_tag_html(html_options)
+        end
+      end
+
       # Creates a scope around a specific model object like form_for, but
       # doesn't create the form tags themselves. This makes fields_for suitable
       # for specifying additional model objects in the same form.
@@ -720,6 +956,62 @@ module ActionView
         capture(builder, &block)
       end
 
+      # Scopes input fields with either an explicit scope or model.
+      # Like +form_with+ does with <tt>:scope</tt> or <tt>:model</tt>,
+      # except it doesn't output the form tags.
+      #
+      #   # Using a scope prefixes the input field names:
+      #   <%= fields :comment do |fields| %>
+      #     <%= fields.text_field :body %>
+      #   <% end %>
+      #   # => <input type="text" name="comment[body] id="comment_body">
+      #
+      #   # Using a model infers the scope and assigns field values:
+      #   <%= fields model: Comment.new(body: "full bodied") do |fields| %<
+      #     <%= fields.text_field :body %>
+      #   <% end %>
+      #   # =>
+      #   <input type="text" name="comment[body] id="comment_body" value="full bodied">
+      #
+      #   # Using +fields+ with +form_with+:
+      #   <%= form_with model: @post do |form| %>
+      #     <%= form.text_field :title %>
+      #
+      #     <%= form.fields :comment do |fields| %>
+      #       <%= fields.text_field :body %>
+      #     <% end %>
+      #   <% end %>
+      #
+      # Much like +form_with+ a FormBuilder instance associated with the scope
+      # or model is yielded, so any generated field names are prefixed with
+      # either the passed scope or the scope inferred from the <tt>:model</tt>.
+      #
+      # === Mixing with other form helpers
+      #
+      # While +form_with+ uses a FormBuilder object it's possible to mix and
+      # match the stand-alone FormHelper methods and methods
+      # from FormTagHelper:
+      #
+      #   <%= fields model: @comment do |fields| %>
+      #     <%= fields.text_field :body %>
+      #
+      #     <%= text_area :commenter, :biography %>
+      #     <%= check_box_tag "comment[all_caps]", "1", @comment.commenter.hulk_mode? %>
+      #   <% end %>
+      #
+      # Same goes for the methods in FormOptionHelper and DateHelper designed
+      # to work with an object as a base, like
+      # FormOptionHelper#collection_select and DateHelper#datetime_select.
+      def fields(scope = nil, model: nil, **options, &block)
+        # TODO: Remove when ids and classes are no longer output by default.
+        if model
+          scope ||= model_name_from_record_or_class(model).param_key
+        end
+
+        builder = instantiate_builder(scope, model, options)
+        capture(builder, &block)
+      end
+
       # Returns a label tag tailored for labelling an input field for a specified attribute (identified by +method+) on an object
       # assigned to the template (identified by +object+). The text of label will default to the attribute name unless a translation
       # is found in the current I18n locale (through helpers.label.<modelname>.<attribute>) or you specify it explicitly.
@@ -1175,6 +1467,32 @@ module ActionView
       end
 
       private
+        def html_options_for_form_with(url_for_options = nil, model = nil, html: {}, local: false,
+          skip_enforcing_utf8: false, **options)
+          html_options = options.except(:index, :include_id, :builder).merge(html)
+          html_options[:method] ||= :patch if model.respond_to?(:persisted?) && model.persisted?
+          html_options[:enforce_utf8] = !skip_enforcing_utf8
+
+          html_options[:enctype] = "multipart/form-data" if html_options.delete(:multipart)
+
+          # The following URL is unescaped, this is just a hash of options, and it is the
+          # responsibility of the caller to escape all the values.
+          html_options[:action] = url_for(url_for_options || {})
+          html_options[:"accept-charset"] = "UTF-8"
+          html_options[:"data-remote"] = true unless local
+
+          if !local && !embed_authenticity_token_in_remote_forms &&
+            html_options[:authenticity_token].blank?
+            # The authenticity token is taken from the meta tag in this case
+            html_options[:authenticity_token] = false
+          elsif html_options[:authenticity_token] == true
+            # Include the default authenticity_token, which is only generated when its set to nil,
+            # but we needed the true value to override the default of no authenticity_token on data-remote.
+            html_options[:authenticity_token] = nil
+          end
+
+          html_options.stringify_keys!
+        end
 
         def instantiate_builder(record_name, record_object, options)
           case record_name
@@ -1183,7 +1501,7 @@ module ActionView
             object_name = record_name
           else
             object = record_name
-            object_name = model_name_from_record_or_class(object).param_key
+            object_name = model_name_from_record_or_class(object).param_key if object
           end
 
           builder = options[:builder] || default_form_builder_class
@@ -1249,7 +1567,7 @@ module ActionView
 
       # The methods which wrap a form helper call.
       class_attribute :field_helpers
-      self.field_helpers = [:fields_for, :label, :text_field, :password_field,
+      self.field_helpers = [:fields_for, :fields, :label, :text_field, :password_field,
                             :hidden_field, :file_field, :text_area, :check_box,
                             :radio_button, :color_field, :search_field,
                             :telephone_field, :phone_field, :date_field,
@@ -1286,6 +1604,9 @@ module ActionView
         @nested_child_index = {}
         @object_name, @object, @template, @options = object_name, object, template, options
         @default_options = @options ? @options.slice(:index, :namespace) : {}
+
+        convert_to_legacy_options(@options)
+
         if @object_name.to_s.match(/\[\]$/)
           if (object ||= @template.instance_variable_get("@#{Regexp.last_match.pre_match}")) && object.respond_to?(:to_param)
             @auto_index = object.to_param
@@ -1293,6 +1614,7 @@ module ActionView
             raise ArgumentError, "object[] naming but object param and @object var don't exist or don't respond to to_param: #{object.inspect}"
           end
         end
+
         @multipart = nil
         @index = options[:index] || options[:child_index]
       end
@@ -1586,6 +1908,13 @@ module ActionView
         @template.fields_for(record_name, record_object, fields_options, &block)
       end
 
+      # See the docs for the <tt>ActionView::FormHelper.fields</tt> helper method.
+      def fields(scope = nil, model: nil, **options, &block)
+        convert_to_legacy_options(options)
+
+        fields_for(scope || model, model, **options, &block)
+      end
+
       # Returns a label tag tailored for labelling an input field for a specified attribute (identified by +method+) on an object
       # assigned to the template (identified by +object+). The text of label will default to the attribute name unless a translation
       # is found in the current I18n locale (through helpers.label.<modelname>.<attribute>) or you specify it explicitly.
@@ -1934,6 +2263,16 @@ module ActionView
           @nested_child_index[name] ||= -1
           @nested_child_index[name] += 1
         end
+
+        def convert_to_legacy_options(options)
+          if options.key?(:skip_id)
+            options[:include_id] = !options.delete(:skip_id)
+          end
+
+          if options.key?(:local)
+            options[:remote] = !options.delete(:local)
+          end
+        end
     end
   end
 
diff --git a/actionview/test/template/form_helper/form_with_test.rb b/actionview/test/template/form_helper/form_with_test.rb
new file mode 100644
index 0000000000..c80a2f61b9
--- /dev/null
+++ b/actionview/test/template/form_helper/form_with_test.rb
@@ -0,0 +1,2134 @@
+require "abstract_unit"
+require "controller/fake_models"
+
+class FormWithTest < ActionView::TestCase
+  include RenderERBUtils
+end
+
+class FormWithActsLikeFormTagTest < FormWithTest
+  tests ActionView::Helpers::FormTagHelper
+
+  setup do
+    @controller = BasicController.new
+  end
+
+  def hidden_fields(options = {})
+    method = options[:method]
+    skip_enforcing_utf8 = options.fetch(:skip_enforcing_utf8, false)
+
+    "".tap do |txt|
+      unless skip_enforcing_utf8
+        txt << %{<input name="utf8" type="hidden" value="&#x2713;" />}
+      end
+
+      if method && !%w(get post).include?(method.to_s)
+        txt << %{<input name="_method" type="hidden" value="#{method}" />}
+      end
+    end
+  end
+
+  def form_text(action = "http://www.example.com", local: false, **options)
+    enctype, html_class, id, method = options.values_at(:enctype, :html_class, :id, :method)
+
+    method = method.to_s == "get" ? "get" : "post"
+
+    txt =  %{<form accept-charset="UTF-8" action="#{action}"}
+    txt << %{ enctype="multipart/form-data"} if enctype
+    txt << %{ data-remote="true"} unless local
+    txt << %{ class="#{html_class}"} if html_class
+    txt << %{ id="#{id}"} if id
+    txt << %{ method="#{method}">}
+  end
+
+  def whole_form(action = "http://www.example.com", options = {})
+    out = form_text(action, options) + hidden_fields(options)
+
+    if block_given?
+      out << yield << "</form>"
+    end
+
+    out
+  end
+
+  def url_for(options)
+    if options.is_a?(Hash)
+      "http://www.example.com"
+    else
+      super
+    end
+  end
+
+  def test_form_with_multipart
+    actual = form_with(multipart: true)
+
+    expected = whole_form("http://www.example.com", enctype: true)
+    assert_dom_equal expected, actual
+  end
+
+  def test_form_with_with_method_patch
+    actual = form_with(method: :patch)
+
+    expected = whole_form("http://www.example.com", method: :patch)
+    assert_dom_equal expected, actual
+  end
+
+  def test_form_with_with_method_put
+    actual = form_with(method: :put)
+
+    expected = whole_form("http://www.example.com", method: :put)
+    assert_dom_equal expected, actual
+  end
+
+  def test_form_with_with_method_delete
+    actual = form_with(method: :delete)
+
+    expected = whole_form("http://www.example.com", method: :delete)
+    assert_dom_equal expected, actual
+  end
+
+  def test_form_with_with_local_true
+    actual = form_with(local: true)
+
+    expected = whole_form("http://www.example.com", local: true)
+    assert_dom_equal expected, actual
+  end
+
+  def test_form_with_skip_enforcing_utf8_true
+    actual = form_with(skip_enforcing_utf8: true)
+    expected = whole_form("http://www.example.com", skip_enforcing_utf8: true)
+    assert_dom_equal expected, actual
+    assert actual.html_safe?
+  end
+
+  def test_form_with_with_block_in_erb
+    output_buffer = render_erb("<%= form_with(url: 'http://www.example.com') do %>Hello world!<% end %>")
+
+    expected = whole_form { "Hello world!" }
+    assert_dom_equal expected, output_buffer
+  end
+
+  def test_form_with_with_block_and_method_in_erb
+    output_buffer = render_erb("<%= form_with(url: 'http://www.example.com', method: :put) do %>Hello world!<% end %>")
+
+    expected = whole_form("http://www.example.com", method: "put") do
+      "Hello world!"
+    end
+
+    assert_dom_equal expected, output_buffer
+  end
+end
+
+class FormWithActsLikeFormForTest < FormWithTest
+  def form_with(*)
+    @output_buffer = super
+  end
+
+  teardown do
+    I18n.backend.reload!
+  end
+
+  setup do
+    # Create "label" locale for testing I18n label helpers
+    I18n.backend.store_translations "label",
+      activemodel: {
+        attributes: {
+          post: {
+            cost: "Total cost"
+          },
+          "post/language": {
+            spanish: "Espanol"
+          }
+        }
+      },
+      helpers: {
+        label: {
+          post: {
+            body: "Write entire text here",
+            color: {
+              red: "Rojo"
+            },
+            comments: {
+              body: "Write body here"
+            }
+          },
+          tag: {
+            value: "Tag"
+          },
+          post_delegate: {
+            title: "Delegate model_name title"
+          }
+        }
+      }
+
+    # Create "submit" locale for testing I18n submit helpers
+    I18n.backend.store_translations "submit",
+      helpers: {
+        submit: {
+          create: "Create %{model}",
+          update: "Confirm %{model} changes",
+          submit: "Save changes",
+          another_post: {
+            update: "Update your %{model}"
+          }
+        }
+      }
+
+    I18n.backend.store_translations "placeholder",
+      activemodel: {
+        attributes: {
+          post: {
+            cost: "Total cost"
+          },
+          "post/cost": {
+            uk: "Pounds"
+          }
+        }
+      },
+      helpers: {
+        placeholder: {
+          post: {
+            title: "What is this about?",
+            written_on: {
+              spanish: "Escrito en"
+            },
+            comments: {
+              body: "Write body here"
+            }
+          },
+          post_delegate: {
+            title: "Delegate model_name title"
+          },
+          tag: {
+            value: "Tag"
+          }
+        }
+      }
+
+    @post = Post.new
+    @comment = Comment.new
+    def @post.errors()
+      Class.new {
+        def [](field); field == "author_name" ? ["can't be empty"] : [] end
+        def empty?() false end
+        def count() 1 end
+        def full_messages() ["Author name can't be empty"] end
+      }.new
+    end
+    def @post.to_key; [123]; end
+    def @post.id; 0; end
+    def @post.id_before_type_cast; "omg"; end
+    def @post.id_came_from_user?; true; end
+    def @post.to_param; "123"; end
+
+    @post.persisted   = true
+    @post.title       = "Hello World"
+    @post.author_name = ""
+    @post.body        = "Back to the hill and over it again!"
+    @post.secret      = 1
+    @post.written_on  = Date.new(2004, 6, 15)
+
+    @post.comments = []
+    @post.comments << @comment
+
+    @post.tags = []
+    @post.tags << Tag.new
+
+    @post_delegator = PostDelegator.new
+
+    @post_delegator.title = "Hello World"
+
+    @car = Car.new("#000FFF")
+  end
+
+  Routes = ActionDispatch::Routing::RouteSet.new
+  Routes.draw do
+    resources :posts do
+      resources :comments
+    end
+
+    namespace :admin do
+      resources :posts do
+        resources :comments
+      end
+    end
+
+    get "/foo", to: "controller#action"
+    root to: "main#index"
+  end
+
+  def _routes
+    Routes
+  end
+
+  include Routes.url_helpers
+
+  def url_for(object)
+    @url_for_options = object
+
+    if object.is_a?(Hash) && object[:use_route].blank? && object[:controller].blank?
+      object.merge!(controller: "main", action: "index")
+    end
+
+    super
+  end
+
+  def test_form_with_requires_arguments
+    error = assert_raises(ArgumentError) do
+      form_for(nil, html: { id: "create-post" }) do
+      end
+    end
+    assert_equal "First argument in form cannot contain nil or be empty", error.message
+
+    error = assert_raises(ArgumentError) do
+      form_for([nil, nil], html: { id: "create-post" }) do
+      end
+    end
+    assert_equal "First argument in form cannot contain nil or be empty", error.message
+  end
+
+  def test_form_with
+    form_with(model: @post, id: "create-post") do |f|
+      concat f.label(:title) { "The Title" }
+      concat f.text_field(:title)
+      concat f.text_area(:body)
+      concat f.check_box(:secret)
+      concat f.submit("Create post")
+      concat f.button("Create post")
+      concat f.button {
+        concat content_tag(:span, "Create post")
+      }
+    end
+
+    expected = whole_form("/posts/123", "create-post", method: "patch") do
+      "<label for='post_title'>The Title</label>" +
+      "<input name='post[title]' type='text' id='post_title' value='Hello World' />" +
+      "<textarea name='post[body]' id='post_body'>\nBack to the hill and over it again!</textarea>" +
+      "<input name='post[secret]' type='hidden' value='0' />" +
+      "<input name='post[secret]' checked='checked' type='checkbox' id='post_secret' value='1' />" +
+      "<input name='commit' data-disable-with='Create post' type='submit' value='Create post' />" +
+      "<button name='button' type='submit'>Create post</button>" +
+      "<button name='button' type='submit'><span>Create post</span></button>"
+    end
+
+    assert_dom_equal expected, output_buffer
+  end
+
+  def test_form_with_with_collection_radio_buttons
+    post = Post.new
+    def post.active; false; end
+    form_with(model: post) do |f|
+      concat f.collection_radio_buttons(:active, [true, false], :to_s, :to_s)
+    end
+
+    expected = whole_form("/posts") do
+      "<input type='hidden' name='post[active]' value='' />" +
+      "<input id='post_active_true' name='post[active]' type='radio' value='true' />" +
+      "<label for='post_active_true'>true</label>" +
+      "<input checked='checked' id='post_active_false' name='post[active]' type='radio' value='false' />" +
+      "<label for='post_active_false'>false</label>"
+    end
+
+    assert_dom_equal expected, output_buffer
+  end
+
+  def test_form_with_with_collection_radio_buttons_with_custom_builder_block
+    post = Post.new
+    def post.active; false; end
+
+    form_with(model: post) do |f|
+      rendered_radio_buttons = f.collection_radio_buttons(:active, [true, false], :to_s, :to_s) do |b|
+        b.label { b.radio_button + b.text }
+      end
+      concat rendered_radio_buttons
+    end
+
+    expected = whole_form("/posts") do
+      "<input type='hidden' name='post[active]' value='' />" +
+      "<label for='post_active_true'>" +
+      "<input id='post_active_true' name='post[active]' type='radio' value='true' />" +
+      "true</label>" +
+      "<label for='post_active_false'>" +
+      "<input checked='checked' id='post_active_false' name='post[active]' type='radio' value='false' />" +
+      "false</label>"
+    end
+
+    assert_dom_equal expected, output_buffer
+  end
+
+  def test_form_with_with_collection_radio_buttons_with_custom_builder_block_does_not_leak_the_template
+    post = Post.new
+    def post.active; false; end
+    def post.id; 1; end
+
+    form_with(model: post) do |f|
+      rendered_radio_buttons = f.collection_radio_buttons(:active, [true, false], :to_s, :to_s) do |b|
+        b.label { b.radio_button + b.text }
+      end
+      concat rendered_radio_buttons
+      concat f.hidden_field :id
+    end
+
+    expected = whole_form("/posts") do
+      "<input type='hidden' name='post[active]' value='' />" +
+      "<label for='post_active_true'>" +
+      "<input id='post_active_true' name='post[active]' type='radio' value='true' />" +
+      "true</label>" +
+      "<label for='post_active_false'>" +
+      "<input checked='checked' id='post_active_false' name='post[active]' type='radio' value='false' />" +
+      "false</label>" +
+      "<input id='post_id' name='post[id]' type='hidden' value='1' />"
+    end
+
+    assert_dom_equal expected, output_buffer
+  end
+
+  def test_form_with_with_index_and_with_collection_radio_buttons
+    post = Post.new
+    def post.active; false; end
+
+    form_with(model: post, index: "1") do |f|
+      concat f.collection_radio_buttons(:active, [true, false], :to_s, :to_s)
+    end
+
+    expected = whole_form("/posts") do
+      "<input type='hidden' name='post[1][active]' value='' />" +
+      "<input id='post_1_active_true' name='post[1][active]' type='radio' value='true' />" +
+      "<label for='post_1_active_true'>true</label>" +
+      "<input checked='checked' id='post_1_active_false' name='post[1][active]' type='radio' value='false' />" +
+      "<label for='post_1_active_false'>false</label>"
+    end
+
+    assert_dom_equal expected, output_buffer
+  end
+
+  def test_form_with_with_collection_check_boxes
+    post = Post.new
+    def post.tag_ids; [1, 3]; end
+    collection = (1..3).map { |i| [i, "Tag #{i}"] }
+    form_with(model: post) do |f|
+      concat f.collection_check_boxes(:tag_ids, collection, :first, :last)
+    end
+
+    expected = whole_form("/posts") do
+      "<input name='post[tag_ids][]' type='hidden' value='' />" +
+      "<input checked='checked' id='post_tag_ids_1' name='post[tag_ids][]' type='checkbox' value='1' />" +
+      "<label for='post_tag_ids_1'>Tag 1</label>" +
+      "<input id='post_tag_ids_2' name='post[tag_ids][]' type='checkbox' value='2' />" +
+      "<label for='post_tag_ids_2'>Tag 2</label>" +
+      "<input checked='checked' id='post_tag_ids_3' name='post[tag_ids][]' type='checkbox' value='3' />" +
+      "<label for='post_tag_ids_3'>Tag 3</label>"
+    end
+
+    assert_dom_equal expected, output_buffer
+  end
+
+  def test_form_with_with_collection_check_boxes_with_custom_builder_block
+    post = Post.new
+    def post.tag_ids; [1, 3]; end
+    collection = (1..3).map { |i| [i, "Tag #{i}"] }
+    form_with(model: post) do |f|
+      rendered_check_boxes = f.collection_check_boxes(:tag_ids, collection, :first, :last) do |b|
+        b.label { b.check_box + b.text }
+      end
+      concat rendered_check_boxes
+    end
+
+    expected = whole_form("/posts") do
+      "<input name='post[tag_ids][]' type='hidden' value='' />" +
+      "<label for='post_tag_ids_1'>" +
+      "<input checked='checked' id='post_tag_ids_1' name='post[tag_ids][]' type='checkbox' value='1' />" +
+      "Tag 1</label>" +
+      "<label for='post_tag_ids_2'>" +
+      "<input id='post_tag_ids_2' name='post[tag_ids][]' type='checkbox' value='2' />" +
+      "Tag 2</label>" +
+      "<label for='post_tag_ids_3'>" +
+      "<input checked='checked' id='post_tag_ids_3' name='post[tag_ids][]' type='checkbox' value='3' />" +
+      "Tag 3</label>"
+    end
+
+    assert_dom_equal expected, output_buffer
+  end
+
+  def test_form_with_with_collection_check_boxes_with_custom_builder_block_does_not_leak_the_template
+    post = Post.new
+    def post.tag_ids; [1, 3]; end
+    def post.id; 1; end
+    collection = (1..3).map { |i| [i, "Tag #{i}"] }
+
+    form_with(model: post) do |f|
+      rendered_check_boxes = f.collection_check_boxes(:tag_ids, collection, :first, :last) do |b|
+        b.label { b.check_box + b.text }
+      end
+      concat rendered_check_boxes
+      concat f.hidden_field :id
+    end
+
+    expected = whole_form("/posts") do
+      "<input name='post[tag_ids][]' type='hidden' value='' />" +
+      "<label for='post_tag_ids_1'>" +
+      "<input checked='checked' id='post_tag_ids_1' name='post[tag_ids][]' type='checkbox' value='1' />" +
+      "Tag 1</label>" +
+      "<label for='post_tag_ids_2'>" +
+      "<input id='post_tag_ids_2' name='post[tag_ids][]' type='checkbox' value='2' />" +
+      "Tag 2</label>" +
+      "<label for='post_tag_ids_3'>" +
+      "<input checked='checked' id='post_tag_ids_3' name='post[tag_ids][]' type='checkbox' value='3' />" +
+      "Tag 3</label>" +
+      "<input id='post_id' name='post[id]' type='hidden' value='1' />"
+    end
+
+    assert_dom_equal expected, output_buffer
+  end
+
+  def test_form_with_index_and_with_collection_check_boxes
+    post = Post.new
+    def post.tag_ids; [1]; end
+    collection = [[1, "Tag 1"]]
+
+    form_with(model: post, index: "1") do |f|
+      concat f.collection_check_boxes(:tag_ids, collection, :first, :last)
+    end
+
+    expected = whole_form("/posts") do
+      "<input name='post[1][tag_ids][]' type='hidden' value='' />" +
+      "<input checked='checked' id='post_1_tag_ids_1' name='post[1][tag_ids][]' type='checkbox' value='1' />" +
+      "<label for='post_1_tag_ids_1'>Tag 1</label>"
+    end
+
+    assert_dom_equal expected, output_buffer
+  end
+
+  def test_form_with_with_file_field_generate_multipart
+    Post.send :attr_accessor, :file
+
+    form_with(model: @post, id: "create-post") do |f|
+      concat f.file_field(:file)
+    end
+
+    expected = whole_form("/posts/123", "create-post", method: "patch", multipart: true) do
+      "<input name='post[file]' type='file' id='post_file' />"
+    end
+
+    assert_dom_equal expected, output_buffer
+  end
+
+  def test_fields_with_file_field_generate_multipart
+    Comment.send :attr_accessor, :file
+
+    form_with(model: @post) do |f|
+      concat f.fields(:comment, model: @post) { |c|
+        concat c.file_field(:file)
+      }
+    end
+
+    expected = whole_form("/posts/123", method: "patch", multipart: true) do
+      "<input name='post[comment][file]' type='file' id='post_comment_file' />"
+    end
+
+    assert_dom_equal expected, output_buffer
+  end
+
+  def test_form_with_with_format
+    form_with(model: @post, format: :json, id: "edit_post_123", class: "edit_post") do |f|
+      concat f.label(:title)
+    end
+
+    expected = whole_form("/posts/123.json", "edit_post_123", "edit_post", method: "patch") do
+      "<label for='post_title'>Title</label>"
+    end
+
+    assert_dom_equal expected, output_buffer
+  end
+
+  def test_form_with_with_format_and_url
+    form_with(model: @post, format: :json, url: "/") do |f|
+      concat f.label(:title)
+    end
+
+    expected = whole_form("/", method: "patch") do
+      "<label for='post_title'>Title</label>"
+    end
+
+    assert_dom_equal expected, output_buffer
+  end
+
+  def test_form_with_with_model_using_relative_model_naming
+    blog_post = Blog::Post.new("And his name will be forty and four.", 44)
+
+    form_with(model: blog_post) do |f|
+      concat f.text_field :title
+      concat f.submit("Edit post")
+    end
+
+    expected = whole_form("/posts/44", method: "patch") do
+      "<input name='post[title]' type='text' id='post_title' value='And his name will be forty and four.' />" +
+      "<input name='commit' data-disable-with='Edit post' type='submit' value='Edit post' />"
+    end
+
+    assert_dom_equal expected, output_buffer
+  end
+
+  def test_form_with_with_symbol_scope
+    form_with(model: @post, scope: "other_name", id: "create-post") do |f|
+      concat f.label(:title, class: "post_title")
+      concat f.text_field(:title)
+      concat f.text_area(:body)
+      concat f.check_box(:secret)
+      concat f.submit("Create post")
+    end
+
+    expected = whole_form("/posts/123", "create-post", method: "patch") do
+      "<label for='other_name_title' class='post_title'>Title</label>" +
+      "<input name='other_name[title]' id='other_name_title' value='Hello World' type='text' />" +
+      "<textarea name='other_name[body]' id='other_name_body'>\nBack to the hill and over it again!</textarea>" +
+      "<input name='other_name[secret]' value='0' type='hidden' />" +
+      "<input name='other_name[secret]' checked='checked' id='other_name_secret' value='1' type='checkbox' />" +
+      "<input name='commit' value='Create post' data-disable-with='Create post' type='submit' />"
+    end
+
+    assert_dom_equal expected, output_buffer
+  end
+
+  def test_form_tags_do_not_call_private_properties_on_form_object
+    obj = Class.new do
+      private def private_property
+        raise "This method should not be called."
+      end
+    end.new
+
+    form_with(model: obj, scope: "other_name", url: "/", id: "edit-other-name") do |f|
+      assert_raise(NoMethodError) { f.hidden_field(:private_property) }
+    end
+  end
+
+  def test_form_with_with_method_as_part_of_html_options
+    form_with(model: @post, url: "/", id: "create-post", html: { method: :delete }) do |f|
+      concat f.text_field(:title)
+      concat f.text_area(:body)
+      concat f.check_box(:secret)
+    end
+
+    expected = whole_form("/", "create-post", method: "delete") do
+      "<input name='post[title]' type='text' id='post_title' value='Hello World' />" +
+      "<textarea name='post[body]' id='post_body'>\nBack to the hill and over it again!</textarea>" +
+      "<input name='post[secret]' type='hidden' value='0' />" +
+      "<input name='post[secret]' checked='checked' type='checkbox' id='post_secret' value='1' />"
+    end
+
+    assert_dom_equal expected, output_buffer
+  end
+
+  def test_form_with_with_method
+    form_with(model: @post, url: "/", method: :delete, id: "create-post") do |f|
+      concat f.text_field(:title)
+      concat f.text_area(:body)
+      concat f.check_box(:secret)
+    end
+
+    expected = whole_form("/", "create-post", method: "delete") do
+      "<input name='post[title]' type='text' id='post_title' value='Hello World' />" +
+      "<textarea name='post[body]' id='post_body'>\nBack to the hill and over it again!</textarea>" +
+      "<input name='post[secret]' type='hidden' value='0' />" +
+      "<input name='post[secret]' checked='checked' type='checkbox' id='post_secret' value='1' />"
+    end
+
+    assert_dom_equal expected, output_buffer
+  end
+
+  def test_form_with_with_search_field
+    # Test case for bug which would emit an "object" attribute
+    # when used with form_for using a search_field form helper
+    form_with(model: Post.new, url: "/search", id: "search-post", method: :get) do |f|
+      concat f.search_field(:title)
+    end
+
+    expected = whole_form("/search", "search-post", method: "get") do
+      "<input name='post[title]' type='search' id='post_title' />"
+    end
+
+    assert_dom_equal expected, output_buffer
+  end
+
+  def test_form_with_enables_remote_by_default
+    form_with(model: @post, url: "/", id: "create-post", method: :patch) do |f|
+      concat f.text_field(:title)
+      concat f.text_area(:body)
+      concat f.check_box(:secret)
+    end
+
+    expected = whole_form("/", "create-post", method: "patch") do
+      "<input name='post[title]' type='text' id='post_title' value='Hello World' />" +
+      "<textarea name='post[body]' id='post_body'>\nBack to the hill and over it again!</textarea>" +
+      "<input name='post[secret]' type='hidden' value='0' />" +
+      "<input name='post[secret]' checked='checked' type='checkbox' id='post_secret' value='1' />"
+    end
+
+    assert_dom_equal expected, output_buffer
+  end
+
+  def test_form_with_skip_enforcing_utf8_true
+    form_with(scope: :post, skip_enforcing_utf8: true) do |f|
+      concat f.text_field(:title)
+    end
+
+    expected = whole_form("/", skip_enforcing_utf8: true) do
+      "<input name='post[title]' type='text' id='post_title' value='Hello World' />"
+    end
+
+    assert_dom_equal expected, output_buffer
+  end
+
+  def test_form_with_skip_enforcing_utf8_false
+    form_with(scope: :post, skip_enforcing_utf8: false) do |f|
+      concat f.text_field(:title)
+    end
+
+    expected = whole_form("/", skip_enforcing_utf8: false) do
+      "<input name='post[title]' type='text' id='post_title' value='Hello World' />"
+    end
+
+    assert_dom_equal expected, output_buffer
+  end
+
+  def test_form_with_without_object
+    form_with(scope: :post, id: "create-post") do |f|
+      concat f.text_field(:title)
+      concat f.text_area(:body)
+      concat f.check_box(:secret)
+    end
+
+    expected = whole_form("/", "create-post") do
+      "<input name='post[title]' type='text' id='post_title' value='Hello World' />" +
+      "<textarea name='post[body]' id='post_body'>\nBack to the hill and over it again!</textarea>" +
+      "<input name='post[secret]' type='hidden' value='0' />" +
+      "<input name='post[secret]' checked='checked' type='checkbox' id='post_secret' value='1' />"
+    end
+
+    assert_dom_equal expected, output_buffer
+  end
+
+  def test_form_with_with_index
+    form_with(model: @post, scope: "post[]") do |f|
+      concat f.label(:title)
+      concat f.text_field(:title)
+      concat f.text_area(:body)
+      concat f.check_box(:secret)
+    end
+
+    expected = whole_form("/posts/123", method: "patch") do
+      "<label for='post_123_title'>Title</label>" +
+      "<input name='post[123][title]' type='text' id='post_123_title' value='Hello World' />" +
+      "<textarea name='post[123][body]' id='post_123_body'>\nBack to the hill and over it again!</textarea>" +
+      "<input name='post[123][secret]' type='hidden' value='0' />" +
+      "<input name='post[123][secret]' checked='checked' type='checkbox' id='post_123_secret' value='1' />"
+    end
+
+    assert_dom_equal expected, output_buffer
+  end
+
+  def test_form_with_with_nil_index_option_override
+    form_with(model: @post, scope: "post[]", index: nil) do |f|
+      concat f.text_field(:title)
+      concat f.text_area(:body)
+      concat f.check_box(:secret)
+    end
+
+    expected = whole_form("/posts/123", method: "patch") do
+      "<input name='post[][title]' type='text' id='post__title' value='Hello World' />" +
+      "<textarea name='post[][body]' id='post__body'>\nBack to the hill and over it again!</textarea>" +
+      "<input name='post[][secret]' type='hidden' value='0' />" +
+      "<input name='post[][secret]' checked='checked' type='checkbox' id='post__secret' value='1' />"
+    end
+
+    assert_dom_equal expected, output_buffer
+  end
+
+  def test_form_with_label_error_wrapping
+    form_with(model: @post) do |f|
+      concat f.label(:author_name, class: "label")
+      concat f.text_field(:author_name)
+      concat f.submit("Create post")
+    end
+
+    expected = whole_form("/posts/123", method: "patch") do
+      "<div class='field_with_errors'><label for='post_author_name' class='label'>Author name</label></div>" +
+      "<div class='field_with_errors'><input name='post[author_name]' type='text' id='post_author_name' value='' /></div>" +
+      "<input name='commit' data-disable-with='Create post' type='submit' value='Create post' />"
+    end
+
+    assert_dom_equal expected, output_buffer
+  end
+
+  def test_form_with_label_error_wrapping_without_conventional_instance_variable
+    post = remove_instance_variable :@post
+
+    form_with(model: post) do |f|
+      concat f.label(:author_name, class: "label")
+      concat f.text_field(:author_name)
+      concat f.submit("Create post")
+    end
+
+    expected = whole_form("/posts/123", method: "patch") do
+      "<div class='field_with_errors'><label for='post_author_name' class='label'>Author name</label></div>" +
+      "<div class='field_with_errors'><input name='post[author_name]' type='text' id='post_author_name' value='' /></div>" +
+      "<input name='commit' data-disable-with='Create post' type='submit' value='Create post' />"
+    end
+
+    assert_dom_equal expected, output_buffer
+  end
+
+  def test_form_with_label_error_wrapping_block_and_non_block_versions
+    form_with(model: @post) do |f|
+      concat f.label(:author_name, "Name", class: "label")
+      concat f.label(:author_name, class: "label") { "Name" }
+    end
+
+    expected = whole_form("/posts/123", method: "patch") do
+      "<div class='field_with_errors'><label for='post_author_name' class='label'>Name</label></div>" +
+      "<div class='field_with_errors'><label for='post_author_name' class='label'>Name</label></div>"
+    end
+
+    assert_dom_equal expected, output_buffer
+  end
+
+  def test_form_with_with_namespace
+    skip "Do namespaces still make sense?"
+    form_for(@post, namespace: "namespace") do |f|
+      concat f.text_field(:title)
+      concat f.text_area(:body)
+      concat f.check_box(:secret)
+    end
+
+    expected = whole_form("/posts/123", "namespace_edit_post_123", "edit_post", method: "patch") do
+      "<input name='post[title]' type='text' id='namespace_post_title' value='Hello World' />" +
+      "<textarea name='post[body]' id='namespace_post_body'>\nBack to the hill and over it again!</textarea>" +
+      "<input name='post[secret]' type='hidden' value='0' />" +
+      "<input name='post[secret]' checked='checked' type='checkbox' id='namespace_post_secret' value='1' />"
+    end
+
+    assert_dom_equal expected, output_buffer
+  end
+
+  def test_submit_with_object_as_new_record_and_locale_strings
+    with_locale :submit do
+      @post.persisted = false
+      @post.stub(:to_key, nil) do
+        form_with(model: @post) do |f|
+          concat f.submit
+        end
+
+        expected = whole_form("/posts") do
+          "<input name='commit' data-disable-with='Create Post' type='submit' value='Create Post' />"
+        end
+
+        assert_dom_equal expected, output_buffer
+      end
+    end
+  end
+
+  def test_submit_with_object_as_existing_record_and_locale_strings
+    with_locale :submit do
+      form_with(model: @post) do |f|
+        concat f.submit
+      end
+
+      expected = whole_form("/posts/123", method: "patch") do
+        "<input name='commit' data-disable-with='Confirm Post changes' type='submit' value='Confirm Post changes' />"
+      end
+
+      assert_dom_equal expected, output_buffer
+    end
+  end
+
+  def test_submit_without_object_and_locale_strings
+    with_locale :submit do
+      form_with(scope: :post) do |f|
+        concat f.submit class: "extra"
+      end
+
+      expected = whole_form do
+        "<input name='commit' class='extra' data-disable-with='Save changes' type='submit' value='Save changes' />"
+      end
+
+      assert_dom_equal expected, output_buffer
+    end
+  end
+
+  def test_submit_with_object_and_nested_lookup
+    with_locale :submit do
+      form_with(model: @post, scope: :another_post) do |f|
+        concat f.submit
+      end
+
+      expected = whole_form("/posts/123", method: "patch") do
+        "<input name='commit' data-disable-with='Update your Post' type='submit' value='Update your Post' />"
+      end
+
+      assert_dom_equal expected, output_buffer
+    end
+  end
+
+  def test_nested_fields
+    @comment.body = "Hello World"
+    form_with(model: @post) do |f|
+      concat f.fields(model: @comment) { |c|
+        concat c.text_field(:body)
+      }
+    end
+
+    expected = whole_form("/posts/123", method: "patch") do
+      "<input name='post[comment][body]' type='text' id='post_comment_body' value='Hello World' />"
+    end
+
+    assert_dom_equal expected, output_buffer
+  end
+
+  def test_deep_nested_fields
+    @comment.save
+    form_with(scope: :posts) do |f|
+      f.fields("post[]", model: @post) do |f2|
+        f2.text_field(:id)
+        @post.comments.each do |comment|
+          concat f2.fields("comment[]", model: comment) { |c|
+            concat c.text_field(:name)
+          }
+        end
+      end
+    end
+
+    expected = whole_form do
+      "<input name='posts[post][0][comment][1][name]' type='text' id='posts_post_0_comment_1_name' value='comment #1' />"
+    end
+
+    assert_dom_equal expected, output_buffer
+  end
+
+  def test_nested_fields_with_nested_collections
+    form_with(model: @post, scope: "post[]") do |f|
+      concat f.text_field(:title)
+      concat f.fields("comment[]", model: @comment) { |c|
+        concat c.text_field(:name)
+      }
+    end
+
+    expected = whole_form("/posts/123", method: "patch") do
+      "<input name='post[123][title]' type='text' id='post_123_title' value='Hello World' />" +
+      "<input name='post[123][comment][][name]' type='text' id='post_123_comment__name' value='new comment' />"
+    end
+
+    assert_dom_equal expected, output_buffer
+  end
+
+  def test_nested_fields_with_index_and_parent_fields
+    form_with(model: @post, index: 1) do |c|
+      concat c.text_field(:title)
+      concat c.fields("comment", model: @comment, index: 1) { |r|
+        concat r.text_field(:name)
+      }
+    end
+
+    expected = whole_form("/posts/123", method: "patch") do
+      "<input name='post[1][title]' type='text' id='post_1_title' value='Hello World' />" +
+      "<input name='post[1][comment][1][name]' type='text' id='post_1_comment_1_name' value='new comment' />"
+    end
+
+    assert_dom_equal expected, output_buffer
+  end
+
+  def test_form_with_with_index_and_nested_fields
+    output_buffer = form_with(model: @post, index: 1) do |f|
+      concat f.fields(:comment, model: @post) { |c|
+        concat c.text_field(:title)
+      }
+    end
+
+    expected = whole_form("/posts/123", method: "patch") do
+      "<input name='post[1][comment][title]' type='text' id='post_1_comment_title' value='Hello World' />"
+    end
+
+    assert_dom_equal expected, output_buffer
+  end
+
+  def test_nested_fields_with_index_on_both
+    form_with(model: @post, index: 1) do |f|
+      concat f.fields(:comment, model: @post, index: 5) { |c|
+        concat c.text_field(:title)
+      }
+    end
+
+    expected = whole_form("/posts/123", method: "patch") do
+      "<input name='post[1][comment][5][title]' type='text' id='post_1_comment_5_title' value='Hello World' />"
+    end
+
+    assert_dom_equal expected, output_buffer
+  end
+
+  def test_nested_fields_with_auto_index
+    form_with(model: @post, scope: "post[]") do |f|
+      concat f.fields(:comment, model: @post) { |c|
+        concat c.text_field(:title)
+      }
+    end
+
+    expected = whole_form("/posts/123", method: "patch") do
+      "<input name='post[123][comment][title]' type='text' id='post_123_comment_title' value='Hello World' />"
+    end
+
+    assert_dom_equal expected, output_buffer
+  end
+
+  def test_nested_fields_with_index_radio_button
+    form_with(model: @post) do |f|
+      concat f.fields(:comment, model: @post, index: 5) { |c|
+        concat c.radio_button(:title, "hello")
+      }
+    end
+
+    expected = whole_form("/posts/123", method: "patch") do
+      "<input name='post[comment][5][title]' type='radio' id='post_comment_5_title_hello' value='hello' />"
+    end
+
+    assert_dom_equal expected, output_buffer
+  end
+
+  def test_nested_fields_with_auto_index_on_both
+    form_with(model: @post, scope: "post[]") do |f|
+      concat f.fields("comment[]", model: @post) { |c|
+        concat c.text_field(:title)
+      }
+    end
+
+    expected = whole_form("/posts/123", method: "patch") do
+      "<input name='post[123][comment][123][title]' type='text' id='post_123_comment_123_title' value='Hello World' />"
+    end
+
+    assert_dom_equal expected, output_buffer
+  end
+
+  def test_nested_fields_with_index_and_auto_index
+    output_buffer = form_with(model: @post, scope: "post[]") do |f|
+      concat f.fields(:comment, model: @post, index: 5) { |c|
+        concat c.text_field(:title)
+      }
+    end
+
+    output_buffer << form_with(model: @post, index: 1) do |f|
+      concat f.fields("comment[]", model: @post) { |c|
+        concat c.text_field(:title)
+      }
+    end
+
+    expected = whole_form("/posts/123", method: "patch") do
+      "<input name='post[123][comment][5][title]' type='text' id='post_123_comment_5_title' value='Hello World' />"
+    end + whole_form("/posts/123", method: "patch") do
+      "<input name='post[1][comment][123][title]' type='text' id='post_1_comment_123_title' value='Hello World' />"
+    end
+
+    assert_dom_equal expected, output_buffer
+  end
+
+  def test_nested_fields_with_a_new_record_on_a_nested_attributes_one_to_one_association
+    @post.author = Author.new
+
+    form_with(model: @post) do |f|
+      concat f.text_field(:title)
+      concat f.fields(:author) { |af|
+        concat af.text_field(:name)
+      }
+    end
+
+    expected = whole_form("/posts/123", method: "patch") do
+      '<input name="post[title]" type="text" id="post_title" value="Hello World" />' +
+        '<input id="post_author_attributes_name" name="post[author_attributes][name]" type="text" value="new author" />'
+    end
+
+    assert_dom_equal expected, output_buffer
+  end
+
+  def test_nested_fields_with_explicitly_passed_object_on_a_nested_attributes_one_to_one_association
+    form_with(model: @post) do |f|
+      f.fields(:author, model: Author.new(123)) do |af|
+        assert_not_nil af.object
+        assert_equal 123, af.object.id
+      end
+    end
+  end
+
+  def test_nested_fields_with_an_existing_record_on_a_nested_attributes_one_to_one_association
+    @post.author = Author.new(321)
+
+    form_with(model: @post) do |f|
+      concat f.text_field(:title)
+      concat f.fields(:author) { |af|
+        concat af.text_field(:name)
+      }
+    end
+
+    expected = whole_form("/posts/123", method: "patch") do
+      '<input name="post[title]" type="text" id="post_title" value="Hello World" />' +
+      '<input id="post_author_attributes_name" name="post[author_attributes][name]" type="text" value="author #321" />' +
+      '<input id="post_author_attributes_id" name="post[author_attributes][id]" type="hidden" value="321" />'
+    end
+
+    assert_dom_equal expected, output_buffer
+  end
+
+  def test_nested_fields_with_an_existing_record_on_a_nested_attributes_one_to_one_association_using_erb_and_inline_block
+    @post.author = Author.new(321)
+
+    form_with(model: @post) do |f|
+      concat f.text_field(:title)
+      concat f.fields(:author) { |af|
+        af.text_field(:name)
+      }
+    end
+
+    expected = whole_form("/posts/123", method: "patch") do
+      '<input name="post[title]" type="text" id="post_title" value="Hello World" />' +
+      '<input id="post_author_attributes_name" name="post[author_attributes][name]" type="text" value="author #321" />' +
+      '<input id="post_author_attributes_id" name="post[author_attributes][id]" type="hidden" value="321" />'
+    end
+
+    assert_dom_equal expected, output_buffer
+  end
+
+  def test_nested_fields_with_an_existing_record_on_a_nested_attributes_one_to_one_association_with_disabled_hidden_id
+    @post.author = Author.new(321)
+
+    form_with(model: @post) do |f|
+      concat f.text_field(:title)
+      concat f.fields(:author, skip_id: true) { |af|
+        af.text_field(:name)
+      }
+    end
+
+    expected = whole_form("/posts/123", method: "patch") do
+      '<input name="post[title]" type="text" id="post_title" value="Hello World" />' +
+      '<input id="post_author_attributes_name" name="post[author_attributes][name]" type="text" value="author #321" />'
+    end
+
+    assert_dom_equal expected, output_buffer
+  end
+
+  def test_nested_fields_with_an_existing_record_on_a_nested_attributes_one_to_one_association_with_disabled_hidden_id_inherited
+    @post.author = Author.new(321)
+
+    form_with(model: @post, skip_id: true) do |f|
+      concat f.text_field(:title)
+      concat f.fields(:author) { |af|
+        af.text_field(:name)
+      }
+    end
+
+    expected = whole_form("/posts/123", method: "patch") do
+      '<input name="post[title]" type="text" id="post_title" value="Hello World" />' +
+      '<input id="post_author_attributes_name" name="post[author_attributes][name]" type="text" value="author #321" />'
+    end
+
+    assert_dom_equal expected, output_buffer
+  end
+
+  def test_nested_fields_with_an_existing_record_on_a_nested_attributes_one_to_one_association_with_disabled_hidden_id_override
+    @post.author = Author.new(321)
+
+    form_with(model: @post, skip_id: true) do |f|
+      concat f.text_field(:title)
+      concat f.fields(:author, skip_id: false) { |af|
+        af.text_field(:name)
+      }
+    end
+
+    expected = whole_form("/posts/123", method: "patch") do
+      '<input name="post[title]" type="text" id="post_title" value="Hello World" />' +
+      '<input id="post_author_attributes_name" name="post[author_attributes][name]" type="text" value="author #321" />' +
+      '<input id="post_author_attributes_id" name="post[author_attributes][id]" type="hidden" value="321" />'
+    end
+
+    assert_dom_equal expected, output_buffer
+  end
+
+  def test_nested_fields_with_existing_records_on_a_nested_attributes_one_to_one_association_with_explicit_hidden_field_placement
+    @post.author = Author.new(321)
+
+    form_with(model: @post) do |f|
+      concat f.text_field(:title)
+      concat f.fields(:author) { |af|
+        concat af.hidden_field(:id)
+        concat af.text_field(:name)
+      }
+    end
+
+    expected = whole_form("/posts/123", method: "patch") do
+      '<input name="post[title]" type="text" id="post_title" value="Hello World" />' +
+      '<input id="post_author_attributes_id" name="post[author_attributes][id]" type="hidden" value="321" />' +
+      '<input id="post_author_attributes_name" name="post[author_attributes][name]" type="text" value="author #321" />'
+    end
+
+    assert_dom_equal expected, output_buffer
+  end
+
+  def test_nested_fields_with_existing_records_on_a_nested_attributes_collection_association
+    @post.comments = Array.new(2) { |id| Comment.new(id + 1) }
+
+    form_with(model: @post) do |f|
+      concat f.text_field(:title)
+      @post.comments.each do |comment|
+        concat f.fields(:comments, model: comment) { |cf|
+          concat cf.text_field(:name)
+        }
+      end
+    end
+
+    expected = whole_form("/posts/123", method: "patch") do
+      '<input name="post[title]" type="text" id="post_title" value="Hello World" />' +
+      '<input id="post_comments_attributes_0_name" name="post[comments_attributes][0][name]" type="text" value="comment #1" />' +
+      '<input id="post_comments_attributes_0_id" name="post[comments_attributes][0][id]" type="hidden" value="1" />' +
+      '<input id="post_comments_attributes_1_name" name="post[comments_attributes][1][name]" type="text" value="comment #2" />' +
+      '<input id="post_comments_attributes_1_id" name="post[comments_attributes][1][id]" type="hidden" value="2" />'
+    end
+
+    assert_dom_equal expected, output_buffer
+  end
+
+  def test_nested_fields_with_existing_records_on_a_nested_attributes_collection_association_with_disabled_hidden_id
+    @post.comments = Array.new(2) { |id| Comment.new(id + 1) }
+    @post.author = Author.new(321)
+
+    form_with(model: @post) do |f|
+      concat f.text_field(:title)
+      concat f.fields(:author) { |af|
+        concat af.text_field(:name)
+      }
+      @post.comments.each do |comment|
+        concat f.fields(:comments, model: comment, skip_id: true) { |cf|
+          concat cf.text_field(:name)
+        }
+      end
+    end
+
+    expected = whole_form("/posts/123", method: "patch") do
+      '<input name="post[title]" type="text" id="post_title" value="Hello World" />' +
+      '<input id="post_author_attributes_name" name="post[author_attributes][name]" type="text" value="author #321" />' +
+      '<input id="post_author_attributes_id" name="post[author_attributes][id]" type="hidden" value="321" />' +
+      '<input id="post_comments_attributes_0_name" name="post[comments_attributes][0][name]" type="text" value="comment #1" />' +
+      '<input id="post_comments_attributes_1_name" name="post[comments_attributes][1][name]" type="text" value="comment #2" />'
+    end
+
+    assert_dom_equal expected, output_buffer
+  end
+
+  def test_nested_fields_with_existing_records_on_a_nested_attributes_collection_association_with_disabled_hidden_id_inherited
+    @post.comments = Array.new(2) { |id| Comment.new(id + 1) }
+    @post.author = Author.new(321)
+
+    form_with(model: @post, skip_id: true) do |f|
+      concat f.text_field(:title)
+      concat f.fields(:author) { |af|
+        concat af.text_field(:name)
+      }
+      @post.comments.each do |comment|
+        concat f.fields(:comments, model: comment) { |cf|
+          concat cf.text_field(:name)
+        }
+      end
+    end
+
+    expected = whole_form("/posts/123", method: "patch") do
+      '<input name="post[title]" type="text" id="post_title" value="Hello World" />' +
+      '<input id="post_author_attributes_name" name="post[author_attributes][name]" type="text" value="author #321" />' +
+      '<input id="post_comments_attributes_0_name" name="post[comments_attributes][0][name]" type="text" value="comment #1" />' +
+      '<input id="post_comments_attributes_1_name" name="post[comments_attributes][1][name]" type="text" value="comment #2" />'
+    end
+
+    assert_dom_equal expected, output_buffer
+  end
+
+  def test_nested_fields_with_existing_records_on_a_nested_attributes_collection_association_with_disabled_hidden_id_override
+    @post.comments = Array.new(2) { |id| Comment.new(id + 1) }
+    @post.author = Author.new(321)
+
+    form_with(model: @post, skip_id: true) do |f|
+      concat f.text_field(:title)
+      concat f.fields(:author, skip_id: false) { |af|
+        concat af.text_field(:name)
+      }
+      @post.comments.each do |comment|
+        concat f.fields(:comments, model: comment) { |cf|
+          concat cf.text_field(:name)
+        }
+      end
+    end
+
+    expected = whole_form("/posts/123", method: "patch") do
+      '<input name="post[title]" type="text" id="post_title" value="Hello World" />' +
+      '<input id="post_author_attributes_name" name="post[author_attributes][name]" type="text" value="author #321" />' +
+      '<input id="post_author_attributes_id" name="post[author_attributes][id]" type="hidden" value="321" />' +
+      '<input id="post_comments_attributes_0_name" name="post[comments_attributes][0][name]" type="text" value="comment #1" />' +
+      '<input id="post_comments_attributes_1_name" name="post[comments_attributes][1][name]" type="text" value="comment #2" />'
+    end
+
+    assert_dom_equal expected, output_buffer
+  end
+
+  def test_nested_fields_with_existing_records_on_a_nested_attributes_collection_association_using_erb_and_inline_block
+    @post.comments = Array.new(2) { |id| Comment.new(id + 1) }
+
+    form_with(model: @post) do |f|
+      concat f.text_field(:title)
+      @post.comments.each do |comment|
+        concat f.fields(:comments, model: comment) { |cf|
+          cf.text_field(:name)
+        }
+      end
+    end
+
+    expected = whole_form("/posts/123", method: "patch") do
+      '<input name="post[title]" type="text" id="post_title" value="Hello World" />' +
+      '<input id="post_comments_attributes_0_name" name="post[comments_attributes][0][name]" type="text" value="comment #1" />' +
+      '<input id="post_comments_attributes_0_id" name="post[comments_attributes][0][id]" type="hidden" value="1" />' +
+      '<input id="post_comments_attributes_1_name" name="post[comments_attributes][1][name]" type="text" value="comment #2" />' +
+      '<input id="post_comments_attributes_1_id" name="post[comments_attributes][1][id]" type="hidden" value="2" />'
+    end
+
+    assert_dom_equal expected, output_buffer
+  end
+
+  def test_nested_fields_with_existing_records_on_a_nested_attributes_collection_association_with_explicit_hidden_field_placement
+    @post.comments = Array.new(2) { |id| Comment.new(id + 1) }
+
+    form_with(model: @post) do |f|
+      concat f.text_field(:title)
+      @post.comments.each do |comment|
+        concat f.fields(:comments, model: comment) { |cf|
+          concat cf.hidden_field(:id)
+          concat cf.text_field(:name)
+        }
+      end
+    end
+
+    expected = whole_form("/posts/123", method: "patch") do
+      '<input name="post[title]" type="text" id="post_title" value="Hello World" />' +
+      '<input id="post_comments_attributes_0_id" name="post[comments_attributes][0][id]" type="hidden" value="1" />' +
+      '<input id="post_comments_attributes_0_name" name="post[comments_attributes][0][name]" type="text" value="comment #1" />' +
+      '<input id="post_comments_attributes_1_id" name="post[comments_attributes][1][id]" type="hidden" value="2" />' +
+      '<input id="post_comments_attributes_1_name" name="post[comments_attributes][1][name]" type="text" value="comment #2" />'
+    end
+
+    assert_dom_equal expected, output_buffer
+  end
+
+  def test_nested_fields_with_new_records_on_a_nested_attributes_collection_association
+    @post.comments = [Comment.new, Comment.new]
+
+    form_with(model: @post) do |f|
+      concat f.text_field(:title)
+      @post.comments.each do |comment|
+        concat f.fields(:comments, model: comment) { |cf|
+          concat cf.text_field(:name)
+        }
+      end
+    end
+
+    expected = whole_form("/posts/123", method: "patch") do
+      '<input name="post[title]" type="text" id="post_title" value="Hello World" />' +
+      '<input id="post_comments_attributes_0_name" name="post[comments_attributes][0][name]" type="text" value="new comment" />' +
+      '<input id="post_comments_attributes_1_name" name="post[comments_attributes][1][name]" type="text" value="new comment" />'
+    end
+
+    assert_dom_equal expected, output_buffer
+  end
+
+  def test_nested_fields_with_existing_and_new_records_on_a_nested_attributes_collection_association
+    @post.comments = [Comment.new(321), Comment.new]
+
+    form_with(model: @post) do |f|
+      concat f.text_field(:title)
+      @post.comments.each do |comment|
+        concat f.fields(:comments, model: comment) { |cf|
+          concat cf.text_field(:name)
+        }
+      end
+    end
+
+    expected = whole_form("/posts/123", method: "patch") do
+      '<input name="post[title]" type="text" id="post_title" value="Hello World" />' +
+      '<input id="post_comments_attributes_0_name" name="post[comments_attributes][0][name]" type="text" value="comment #321" />' +
+      '<input id="post_comments_attributes_0_id" name="post[comments_attributes][0][id]" type="hidden" value="321" />' +
+      '<input id="post_comments_attributes_1_name" name="post[comments_attributes][1][name]" type="text" value="new comment" />'
+    end
+
+    assert_dom_equal expected, output_buffer
+  end
+
+  def test_nested_fields_with_an_empty_supplied_attributes_collection
+    form_with(model: @post) do |f|
+      concat f.text_field(:title)
+      f.fields(:comments, model: []) do |cf|
+        concat cf.text_field(:name)
+      end
+    end
+
+    expected = whole_form("/posts/123", method: "patch") do
+      '<input name="post[title]" type="text" id="post_title" value="Hello World" />'
+    end
+
+    assert_dom_equal expected, output_buffer
+  end
+
+  def test_nested_fields_with_existing_records_on_a_supplied_nested_attributes_collection
+    @post.comments = Array.new(2) { |id| Comment.new(id + 1) }
+
+    form_with(model: @post) do |f|
+      concat f.text_field(:title)
+      concat f.fields(:comments, model: @post.comments) { |cf|
+        concat cf.text_field(:name)
+      }
+    end
+
+    expected = whole_form("/posts/123", method: "patch") do
+      '<input name="post[title]" type="text" id="post_title" value="Hello World" />' +
+      '<input id="post_comments_attributes_0_name" name="post[comments_attributes][0][name]" type="text" value="comment #1" />' +
+      '<input id="post_comments_attributes_0_id" name="post[comments_attributes][0][id]" type="hidden" value="1" />' +
+      '<input id="post_comments_attributes_1_name" name="post[comments_attributes][1][name]" type="text" value="comment #2" />' +
+      '<input id="post_comments_attributes_1_id" name="post[comments_attributes][1][id]" type="hidden" value="2" />'
+    end
+
+    assert_dom_equal expected, output_buffer
+  end
+
+  def test_nested_fields_arel_like
+    @post.comments = ArelLike.new
+
+    form_with(model: @post) do |f|
+      concat f.text_field(:title)
+      concat f.fields(:comments, model: @post.comments) { |cf|
+        concat cf.text_field(:name)
+      }
+    end
+
+    expected = whole_form("/posts/123", method: "patch") do
+      '<input name="post[title]" type="text" id="post_title" value="Hello World" />' +
+      '<input id="post_comments_attributes_0_name" name="post[comments_attributes][0][name]" type="text" value="comment #1" />' +
+      '<input id="post_comments_attributes_0_id" name="post[comments_attributes][0][id]" type="hidden" value="1" />' +
+      '<input id="post_comments_attributes_1_name" name="post[comments_attributes][1][name]" type="text" value="comment #2" />' +
+      '<input id="post_comments_attributes_1_id" name="post[comments_attributes][1][id]" type="hidden" value="2" />'
+    end
+
+    assert_dom_equal expected, output_buffer
+  end
+
+  def test_nested_fields_label_translation_with_more_than_10_records
+    @post.comments = Array.new(11) { |id| Comment.new(id + 1) }
+
+    params = 11.times.map { ["post.comments.body", default: [:"comment.body", ""], scope: "helpers.label"] }
+    assert_called_with(I18n, :t, params, returns: "Write body here") do
+      form_with(model: @post) do |f|
+        f.fields(:comments) do |cf|
+          concat cf.label(:body)
+        end
+      end
+    end
+  end
+
+  def test_nested_fields_with_existing_records_on_a_supplied_nested_attributes_collection_different_from_record_one
+    comments = Array.new(2) { |id| Comment.new(id + 1) }
+    @post.comments = []
+
+    form_with(model: @post) do |f|
+      concat f.text_field(:title)
+      concat f.fields(:comments, model: comments) { |cf|
+        concat cf.text_field(:name)
+      }
+    end
+
+    expected = whole_form("/posts/123", method: "patch") do
+      '<input name="post[title]" type="text" id="post_title" value="Hello World" />' +
+      '<input id="post_comments_attributes_0_name" name="post[comments_attributes][0][name]" type="text" value="comment #1" />' +
+      '<input id="post_comments_attributes_0_id" name="post[comments_attributes][0][id]" type="hidden" value="1" />' +
+      '<input id="post_comments_attributes_1_name" name="post[comments_attributes][1][name]" type="text" value="comment #2" />' +
+      '<input id="post_comments_attributes_1_id" name="post[comments_attributes][1][id]" type="hidden" value="2" />'
+    end
+
+    assert_dom_equal expected, output_buffer
+  end
+
+  def test_nested_fields_on_a_nested_attributes_collection_association_yields_only_builder
+    @post.comments = [Comment.new(321), Comment.new]
+    yielded_comments = []
+
+    form_with(model: @post) do |f|
+      concat f.text_field(:title)
+      concat f.fields(:comments) { |cf|
+        concat cf.text_field(:name)
+        yielded_comments << cf.object
+      }
+    end
+
+    expected = whole_form("/posts/123", method: "patch") do
+      '<input name="post[title]" type="text" id="post_title" value="Hello World" />' +
+      '<input id="post_comments_attributes_0_name" name="post[comments_attributes][0][name]" type="text" value="comment #321" />' +
+      '<input id="post_comments_attributes_0_id" name="post[comments_attributes][0][id]" type="hidden" value="321" />' +
+      '<input id="post_comments_attributes_1_name" name="post[comments_attributes][1][name]" type="text" value="new comment" />'
+    end
+
+    assert_dom_equal expected, output_buffer
+    assert_equal yielded_comments, @post.comments
+  end
+
+  def test_nested_fields_with_child_index_option_override_on_a_nested_attributes_collection_association
+    @post.comments = []
+
+    form_with(model: @post) do |f|
+      concat f.fields(:comments, model: Comment.new(321), child_index: "abc") { |cf|
+        concat cf.text_field(:name)
+      }
+    end
+
+    expected = whole_form("/posts/123", method: "patch") do
+      '<input id="post_comments_attributes_abc_name" name="post[comments_attributes][abc][name]" type="text" value="comment #321" />' +
+      '<input id="post_comments_attributes_abc_id" name="post[comments_attributes][abc][id]" type="hidden" value="321" />'
+    end
+
+    assert_dom_equal expected, output_buffer
+  end
+
+  def test_nested_fields_with_child_index_as_lambda_option_override_on_a_nested_attributes_collection_association
+    @post.comments = []
+
+    form_with(model: @post) do |f|
+      concat f.fields(:comments, model: Comment.new(321), child_index: -> { "abc" }) { |cf|
+        concat cf.text_field(:name)
+      }
+    end
+
+    expected = whole_form("/posts/123", method: "patch") do
+      '<input id="post_comments_attributes_abc_name" name="post[comments_attributes][abc][name]" type="text" value="comment #321" />' +
+      '<input id="post_comments_attributes_abc_id" name="post[comments_attributes][abc][id]" type="hidden" value="321" />'
+    end
+
+    assert_dom_equal expected, output_buffer
+  end
+
+  class FakeAssociationProxy
+    def to_ary
+      [1, 2, 3]
+    end
+  end
+
+  def test_nested_fields_with_child_index_option_override_on_a_nested_attributes_collection_association_with_proxy
+    @post.comments = FakeAssociationProxy.new
+
+    form_with(model: @post) do |f|
+      concat f.fields(:comments, model: Comment.new(321), child_index: "abc") { |cf|
+        concat cf.text_field(:name)
+      }
+    end
+
+    expected = whole_form("/posts/123", method: "patch") do
+      '<input id="post_comments_attributes_abc_name" name="post[comments_attributes][abc][name]" type="text" value="comment #321" />' +
+      '<input id="post_comments_attributes_abc_id" name="post[comments_attributes][abc][id]" type="hidden" value="321" />'
+    end
+
+    assert_dom_equal expected, output_buffer
+  end
+
+  def test_nested_fields_index_method_with_existing_records_on_a_nested_attributes_collection_association
+    @post.comments = Array.new(2) { |id| Comment.new(id + 1) }
+
+    form_with(model: @post) do |f|
+      expected = 0
+      @post.comments.each do |comment|
+        f.fields(:comments, model: comment) { |cf|
+          assert_equal cf.index, expected
+          expected += 1
+        }
+      end
+    end
+  end
+
+  def test_nested_fields_index_method_with_existing_and_new_records_on_a_nested_attributes_collection_association
+    @post.comments = [Comment.new(321), Comment.new]
+
+    form_with(model: @post) do |f|
+      expected = 0
+      @post.comments.each do |comment|
+        f.fields(:comments, model: comment) { |cf|
+          assert_equal cf.index, expected
+          expected += 1
+        }
+      end
+    end
+  end
+
+  def test_nested_fields_index_method_with_existing_records_on_a_supplied_nested_attributes_collection
+    @post.comments = Array.new(2) { |id| Comment.new(id + 1) }
+
+    form_with(model: @post) do |f|
+      expected = 0
+      f.fields(:comments, model: @post.comments) { |cf|
+        assert_equal cf.index, expected
+        expected += 1
+      }
+    end
+  end
+
+  def test_nested_fields_index_method_with_child_index_option_override_on_a_nested_attributes_collection_association
+    @post.comments = []
+
+    form_with(model: @post) do |f|
+      f.fields(:comments, model: Comment.new(321), child_index: "abc") { |cf|
+        assert_equal cf.index, "abc"
+      }
+    end
+  end
+
+  def test_nested_fields_uses_unique_indices_for_different_collection_associations
+    @post.comments = [Comment.new(321)]
+    @post.tags = [Tag.new(123), Tag.new(456)]
+    @post.comments[0].relevances = []
+    @post.tags[0].relevances = []
+    @post.tags[1].relevances = []
+
+    form_with(model: @post) do |f|
+      concat f.fields(:comments, model: @post.comments[0]) { |cf|
+        concat cf.text_field(:name)
+        concat cf.fields(:relevances, model: CommentRelevance.new(314)) { |crf|
+          concat crf.text_field(:value)
+        }
+      }
+      concat f.fields(:tags, model: @post.tags[0]) { |tf|
+        concat tf.text_field(:value)
+        concat tf.fields(:relevances, model: TagRelevance.new(3141)) { |trf|
+          concat trf.text_field(:value)
+        }
+      }
+      concat f.fields("tags", model: @post.tags[1]) { |tf|
+        concat tf.text_field(:value)
+        concat tf.fields(:relevances, model: TagRelevance.new(31415)) { |trf|
+          concat trf.text_field(:value)
+        }
+      }
+    end
+
+    expected = whole_form("/posts/123", method: "patch") do
+      '<input id="post_comments_attributes_0_name" name="post[comments_attributes][0][name]" type="text" value="comment #321" />' +
+      '<input id="post_comments_attributes_0_relevances_attributes_0_value" name="post[comments_attributes][0][relevances_attributes][0][value]" type="text" value="commentrelevance #314" />' +
+      '<input id="post_comments_attributes_0_relevances_attributes_0_id" name="post[comments_attributes][0][relevances_attributes][0][id]" type="hidden" value="314" />' +
+      '<input id="post_comments_attributes_0_id" name="post[comments_attributes][0][id]" type="hidden" value="321" />' +
+      '<input id="post_tags_attributes_0_value" name="post[tags_attributes][0][value]" type="text" value="tag #123" />' +
+      '<input id="post_tags_attributes_0_relevances_attributes_0_value" name="post[tags_attributes][0][relevances_attributes][0][value]" type="text" value="tagrelevance #3141" />' +
+      '<input id="post_tags_attributes_0_relevances_attributes_0_id" name="post[tags_attributes][0][relevances_attributes][0][id]" type="hidden" value="3141" />' +
+      '<input id="post_tags_attributes_0_id" name="post[tags_attributes][0][id]" type="hidden" value="123" />' +
+      '<input id="post_tags_attributes_1_value" name="post[tags_attributes][1][value]" type="text" value="tag #456" />' +
+      '<input id="post_tags_attributes_1_relevances_attributes_0_value" name="post[tags_attributes][1][relevances_attributes][0][value]" type="text" value="tagrelevance #31415" />' +
+      '<input id="post_tags_attributes_1_relevances_attributes_0_id" name="post[tags_attributes][1][relevances_attributes][0][id]" type="hidden" value="31415" />' +
+      '<input id="post_tags_attributes_1_id" name="post[tags_attributes][1][id]" type="hidden" value="456" />'
+    end
+
+    assert_dom_equal expected, output_buffer
+  end
+
+  def test_nested_fields_with_hash_like_model
+    @author = HashBackedAuthor.new
+
+    form_with(model: @post) do |f|
+      concat f.fields(:author, model: @author) { |af|
+        concat af.text_field(:name)
+      }
+    end
+
+    expected = whole_form("/posts/123", method: "patch") do
+      '<input id="post_author_attributes_name" name="post[author_attributes][name]" type="text" value="hash backed author" />'
+    end
+
+    assert_dom_equal expected, output_buffer
+  end
+
+  def test_fields
+    output_buffer = fields(:post, model: @post) do |f|
+      concat f.text_field(:title)
+      concat f.text_area(:body)
+      concat f.check_box(:secret)
+    end
+
+    expected =
+      "<input name='post[title]' type='text' id='post_title' value='Hello World' />" +
+      "<textarea name='post[body]' id='post_body'>\nBack to the hill and over it again!</textarea>" +
+      "<input name='post[secret]' type='hidden' value='0' />" +
+      "<input name='post[secret]' checked='checked' type='checkbox' id='post_secret' value='1' />"
+
+    assert_dom_equal expected, output_buffer
+  end
+
+  def test_fields_with_index
+    output_buffer = fields("post[]", model: @post) do |f|
+      concat f.text_field(:title)
+      concat f.text_area(:body)
+      concat f.check_box(:secret)
+    end
+
+    expected =
+      "<input name='post[123][title]' type='text' id='post_123_title' value='Hello World' />" +
+      "<textarea name='post[123][body]' id='post_123_body'>\nBack to the hill and over it again!</textarea>" +
+      "<input name='post[123][secret]' type='hidden' value='0' />" +
+      "<input name='post[123][secret]' checked='checked' type='checkbox' id='post_123_secret' value='1' />"
+
+    assert_dom_equal expected, output_buffer
+  end
+
+  def test_fields_with_nil_index_option_override
+    output_buffer = fields("post[]", model: @post, index: nil) do |f|
+      concat f.text_field(:title)
+      concat f.text_area(:body)
+      concat f.check_box(:secret)
+    end
+
+    expected =
+      "<input name='post[][title]' type='text' id='post__title' value='Hello World' />" +
+      "<textarea name='post[][body]' id='post__body'>\nBack to the hill and over it again!</textarea>" +
+      "<input name='post[][secret]' type='hidden' value='0' />" +
+      "<input name='post[][secret]' checked='checked' type='checkbox' id='post__secret' value='1' />"
+
+    assert_dom_equal expected, output_buffer
+  end
+
+  def test_fields_with_index_option_override
+    output_buffer = fields("post[]", model: @post, index: "abc") do |f|
+      concat f.text_field(:title)
+      concat f.text_area(:body)
+      concat f.check_box(:secret)
+    end
+
+    expected =
+      "<input name='post[abc][title]' type='text' id='post_abc_title' value='Hello World' />" +
+      "<textarea name='post[abc][body]' id='post_abc_body'>\nBack to the hill and over it again!</textarea>" +
+      "<input name='post[abc][secret]' type='hidden' value='0' />" +
+      "<input name='post[abc][secret]' checked='checked' type='checkbox' id='post_abc_secret' value='1' />"
+
+    assert_dom_equal expected, output_buffer
+  end
+
+  def test_fields_without_object
+    output_buffer = fields(:post) do |f|
+      concat f.text_field(:title)
+      concat f.text_area(:body)
+      concat f.check_box(:secret)
+    end
+
+    expected =
+      "<input name='post[title]' type='text' id='post_title' value='Hello World' />" +
+      "<textarea name='post[body]' id='post_body'>\nBack to the hill and over it again!</textarea>" +
+      "<input name='post[secret]' type='hidden' value='0' />" +
+      "<input name='post[secret]' checked='checked' type='checkbox' id='post_secret' value='1' />"
+
+    assert_dom_equal expected, output_buffer
+  end
+
+  def test_fields_with_only_object
+    output_buffer = fields(model: @post) do |f|
+      concat f.text_field(:title)
+      concat f.text_area(:body)
+      concat f.check_box(:secret)
+    end
+
+    expected =
+      "<input name='post[title]' type='text' id='post_title' value='Hello World' />" +
+      "<textarea name='post[body]' id='post_body'>\nBack to the hill and over it again!</textarea>" +
+      "<input name='post[secret]' type='hidden' value='0' />" +
+      "<input name='post[secret]' checked='checked' type='checkbox' id='post_secret' value='1' />"
+
+    assert_dom_equal expected, output_buffer
+  end
+
+  def test_fields_object_with_bracketed_name
+    output_buffer = fields("author[post]", model: @post) do |f|
+      concat f.label(:title)
+      concat f.text_field(:title)
+    end
+
+    assert_dom_equal "<label for=\"author_post_title\">Title</label>" +
+    "<input name='author[post][title]' type='text' id='author_post_title' value='Hello World' />",
+      output_buffer
+  end
+
+  def test_fields_object_with_bracketed_name_and_index
+    output_buffer = fields("author[post]", model: @post, index: 1) do |f|
+      concat f.label(:title)
+      concat f.text_field(:title)
+    end
+
+    assert_dom_equal "<label for=\"author_post_1_title\">Title</label>" +
+      "<input name='author[post][1][title]' type='text' id='author_post_1_title' value='Hello World' />",
+      output_buffer
+  end
+
+  def test_form_builder_does_not_have_form_with_method
+    assert_not_includes ActionView::Helpers::FormBuilder.instance_methods, :form_with
+  end
+
+  def test_form_with_and_fields
+    form_with(model: @post, scope: :post, id: "create-post") do |post_form|
+      concat post_form.text_field(:title)
+      concat post_form.text_area(:body)
+
+      concat fields(:parent_post, model: @post) { |parent_fields|
+        concat parent_fields.check_box(:secret)
+      }
+    end
+
+    expected = whole_form("/posts/123", "create-post", method: "patch") do
+      "<input name='post[title]' type='text' id='post_title' value='Hello World' />" +
+      "<textarea name='post[body]' id='post_body'>\nBack to the hill and over it again!</textarea>" +
+      "<input name='parent_post[secret]' type='hidden' value='0' />" +
+      "<input name='parent_post[secret]' checked='checked' type='checkbox' id='parent_post_secret' value='1' />"
+    end
+
+    assert_dom_equal expected, output_buffer
+  end
+
+  def test_form_with_and_fields_with_object
+    form_with(model: @post, scope: :post, id: "create-post") do |post_form|
+      concat post_form.text_field(:title)
+      concat post_form.text_area(:body)
+
+      concat post_form.fields(model: @comment) { |comment_fields|
+        concat comment_fields.text_field(:name)
+      }
+    end
+
+    expected = whole_form("/posts/123", "create-post", method: "patch") do
+      "<input name='post[title]' type='text' id='post_title' value='Hello World' />" +
+      "<textarea name='post[body]' id='post_body'>\nBack to the hill and over it again!</textarea>" +
+      "<input name='post[comment][name]' type='text' id='post_comment_name' value='new comment' />"
+    end
+
+    assert_dom_equal expected, output_buffer
+  end
+
+  def test_form_with_and_fields_with_non_nested_association_and_without_object
+    form_with(model: @post) do |f|
+      concat f.fields(:category) { |c|
+        concat c.text_field(:name)
+      }
+    end
+
+    expected = whole_form("/posts/123", method: "patch") do
+      "<input name='post[category][name]' type='text' id='post_category_name' />"
+    end
+
+    assert_dom_equal expected, output_buffer
+  end
+
+  class LabelledFormBuilder < ActionView::Helpers::FormBuilder
+    (field_helpers - %w(hidden_field)).each do |selector|
+      class_eval <<-RUBY_EVAL, __FILE__, __LINE__ + 1
+        def #{selector}(field, *args, &proc)
+          ("<label for='\#{field}'>\#{field.to_s.humanize}:</label> " + super + "<br/>").html_safe
+        end
+      RUBY_EVAL
+    end
+  end
+
+  def test_form_with_with_labelled_builder
+    form_with(model: @post, builder: LabelledFormBuilder) do |f|
+      concat f.text_field(:title)
+      concat f.text_area(:body)
+      concat f.check_box(:secret)
+    end
+
+    expected = whole_form("/posts/123", method: "patch") do
+      "<label for='title'>Title:</label> <input name='post[title]' type='text' id='post_title' value='Hello World' /><br/>" +
+      "<label for='body'>Body:</label> <textarea name='post[body]' id='post_body'>\nBack to the hill and over it again!</textarea><br/>" +
+      "<label for='secret'>Secret:</label> <input name='post[secret]' type='hidden' value='0' /><input name='post[secret]' checked='checked' type='checkbox' id='post_secret' value='1' /><br/>"
+    end
+
+    assert_dom_equal expected, output_buffer
+  end
+
+  def test_default_form_builder
+    old_default_form_builder, ActionView::Base.default_form_builder =
+      ActionView::Base.default_form_builder, LabelledFormBuilder
+
+    form_with(model: @post) do |f|
+      concat f.text_field(:title)
+      concat f.text_area(:body)
+      concat f.check_box(:secret)
+    end
+
+    expected = whole_form("/posts/123", method: "patch") do
+      "<label for='title'>Title:</label> <input name='post[title]' type='text' id='post_title' value='Hello World' /><br/>" +
+      "<label for='body'>Body:</label> <textarea name='post[body]' id='post_body'>\nBack to the hill and over it again!</textarea><br/>" +
+      "<label for='secret'>Secret:</label> <input name='post[secret]' type='hidden' value='0' /><input name='post[secret]' checked='checked' type='checkbox' id='post_secret' value='1' /><br/>"
+    end
+
+    assert_dom_equal expected, output_buffer
+  ensure
+    ActionView::Base.default_form_builder = old_default_form_builder
+  end
+
+  def test_lazy_loading_default_form_builder
+    old_default_form_builder, ActionView::Base.default_form_builder =
+      ActionView::Base.default_form_builder, "FormWithActsLikeFormForTest::LabelledFormBuilder"
+
+    form_with(model: @post) do |f|
+      concat f.text_field(:title)
+    end
+
+    expected = whole_form("/posts/123", method: "patch") do
+      "<label for='title'>Title:</label> <input name='post[title]' type='text' id='post_title' value='Hello World' /><br/>"
+    end
+
+    assert_dom_equal expected, output_buffer
+  ensure
+    ActionView::Base.default_form_builder = old_default_form_builder
+  end
+
+  def test_form_builder_override
+    self.default_form_builder = LabelledFormBuilder
+
+    output_buffer = fields(:post, model: @post) do |f|
+      concat f.text_field(:title)
+    end
+
+    expected = "<label for='title'>Title:</label> <input name='post[title]' type='text' id='post_title' value='Hello World' /><br/>"
+
+    assert_dom_equal expected, output_buffer
+  end
+
+  def test_lazy_loading_form_builder_override
+    self.default_form_builder = "FormWithActsLikeFormForTest::LabelledFormBuilder"
+
+    output_buffer = fields(:post, model: @post) do |f|
+      concat f.text_field(:title)
+    end
+
+    expected = "<label for='title'>Title:</label> <input name='post[title]' type='text' id='post_title' value='Hello World' /><br/>"
+
+    assert_dom_equal expected, output_buffer
+  end
+
+  def test_fields_with_labelled_builder
+    output_buffer = fields(:post, model: @post, builder: LabelledFormBuilder) do |f|
+      concat f.text_field(:title)
+      concat f.text_area(:body)
+      concat f.check_box(:secret)
+    end
+
+    expected =
+      "<label for='title'>Title:</label> <input name='post[title]' type='text' id='post_title' value='Hello World' /><br/>" +
+      "<label for='body'>Body:</label> <textarea name='post[body]' id='post_body'>\nBack to the hill and over it again!</textarea><br/>" +
+      "<label for='secret'>Secret:</label> <input name='post[secret]' type='hidden' value='0' /><input name='post[secret]' checked='checked' type='checkbox' id='post_secret' value='1' /><br/>"
+
+    assert_dom_equal expected, output_buffer
+  end
+
+  def test_form_with_with_labelled_builder_with_nested_fields_without_options_hash
+    klass = nil
+
+    form_with(model: @post, builder: LabelledFormBuilder) do |f|
+      f.fields(:comments, model: Comment.new) do |nested_fields|
+        klass = nested_fields.class
+        ""
+      end
+    end
+
+    assert_equal LabelledFormBuilder, klass
+  end
+
+  def test_form_with_with_labelled_builder_with_nested_fields_with_options_hash
+    klass = nil
+
+    form_with(model: @post, builder: LabelledFormBuilder) do |f|
+      f.fields(:comments, model: Comment.new, index: "foo") do |nested_fields|
+        klass = nested_fields.class
+        ""
+      end
+    end
+
+    assert_equal LabelledFormBuilder, klass
+  end
+
+  def test_form_with_with_labelled_builder_path
+    path = nil
+
+    form_with(model: @post, builder: LabelledFormBuilder) do |f|
+      path = f.to_partial_path
+      ""
+    end
+
+    assert_equal "labelled_form", path
+  end
+
+  class LabelledFormBuilderSubclass < LabelledFormBuilder; end
+
+  def test_form_with_with_labelled_builder_with_nested_fields_with_custom_builder
+    klass = nil
+
+    form_with(model: @post, builder: LabelledFormBuilder) do |f|
+      f.fields(:comments, model: Comment.new, builder: LabelledFormBuilderSubclass) do |nested_fields|
+        klass = nested_fields.class
+        ""
+      end
+    end
+
+    assert_equal LabelledFormBuilderSubclass, klass
+  end
+
+  def test_form_with_with_html_options_adds_options_to_form_tag
+    form_with(model: @post, html: { id: "some_form", class: "some_class", multipart: true }) do |f| end
+    expected = whole_form("/posts/123", "some_form", "some_class", method: "patch", multipart: "multipart/form-data")
+
+    assert_dom_equal expected, output_buffer
+  end
+
+  def test_form_with_with_string_url_option
+    form_with(model: @post, url: "http://www.otherdomain.com") do |f| end
+
+    assert_dom_equal whole_form("http://www.otherdomain.com", method: "patch"), output_buffer
+  end
+
+  def test_form_with_with_hash_url_option
+    form_with(model: @post, url: { controller: "controller", action: "action" }) do |f| end
+
+    assert_equal "controller", @url_for_options[:controller]
+    assert_equal "action", @url_for_options[:action]
+  end
+
+  def test_form_with_with_record_url_option
+    form_with(model: @post, url: @post) do |f| end
+
+    expected = whole_form("/posts/123", method: "patch")
+    assert_dom_equal expected, output_buffer
+  end
+
+  def test_form_with_with_existing_object
+    form_with(model: @post) do |f| end
+
+    expected = whole_form("/posts/123", method: "patch")
+    assert_dom_equal expected, output_buffer
+  end
+
+  def test_form_with_with_new_object
+    post = Post.new
+    post.persisted = false
+    def post.to_key; nil; end
+
+    form_with(model: post) {}
+
+    expected = whole_form("/posts")
+    assert_dom_equal expected, output_buffer
+  end
+
+  def test_form_with_with_existing_object_in_list
+    @comment.save
+    form_with(model: [@post, @comment]) {}
+
+    expected = whole_form(post_comment_path(@post, @comment), method: "patch")
+    assert_dom_equal expected, output_buffer
+  end
+
+  def test_form_with_with_new_object_in_list
+    form_with(model: [@post, @comment]) {}
+
+    expected = whole_form(post_comments_path(@post))
+    assert_dom_equal expected, output_buffer
+  end
+
+  def test_form_with_with_existing_object_and_namespace_in_list
+    @comment.save
+    form_with(model: [:admin, @post, @comment]) {}
+
+    expected = whole_form(admin_post_comment_path(@post, @comment), method: "patch")
+    assert_dom_equal expected, output_buffer
+  end
+
+  def test_form_with_with_new_object_and_namespace_in_list
+    form_with(model: [:admin, @post, @comment]) {}
+
+    expected = whole_form(admin_post_comments_path(@post))
+    assert_dom_equal expected, output_buffer
+  end
+
+  def test_form_with_with_existing_object_and_custom_url
+    form_with(model: @post, url: "/super_posts") do |f| end
+
+    expected = whole_form("/super_posts", method: "patch")
+    assert_dom_equal expected, output_buffer
+  end
+
+  def test_form_with_with_default_method_as_patch
+    form_with(model: @post) {}
+    expected = whole_form("/posts/123", method: "patch")
+    assert_dom_equal expected, output_buffer
+  end
+
+  def test_form_with_with_data_attributes
+    form_with(model: @post, data: { behavior: "stuff" }) {}
+    assert_match %r|data-behavior="stuff"|, output_buffer
+    assert_match %r|data-remote="true"|, output_buffer
+  end
+
+  def test_fields_returns_block_result
+    output = fields(model: Post.new) { |f| "fields" }
+    assert_equal "fields", output
+  end
+
+  def test_form_with_only_instantiates_builder_once
+    initialization_count = 0
+    builder_class = Class.new(ActionView::Helpers::FormBuilder) do
+      define_method :initialize do |*args|
+        super(*args)
+        initialization_count += 1
+      end
+    end
+
+    form_with(model: @post, builder: builder_class) {}
+    assert_equal 1, initialization_count, "form builder instantiated more than once"
+  end
+
+  protected
+    def hidden_fields(options = {})
+      method = options[:method]
+
+      if options.fetch(:skip_enforcing_utf8, false)
+        txt = ""
+      else
+        txt = %{<input name="utf8" type="hidden" value="&#x2713;" />}
+      end
+
+      if method && !%w(get post).include?(method.to_s)
+        txt << %{<input name="_method" type="hidden" value="#{method}" />}
+      end
+
+      txt
+    end
+
+    def form_text(action = "/", id = nil, html_class = nil, local = nil, multipart = nil, method = nil)
+      txt =  %{<form accept-charset="UTF-8" action="#{action}"}
+      txt << %{ enctype="multipart/form-data"} if multipart
+      txt << %{ data-remote="true"} unless local
+      txt << %{ class="#{html_class}"} if html_class
+      txt << %{ id="#{id}"} if id
+      method = method.to_s == "get" ? "get" : "post"
+      txt << %{ method="#{method}">}
+    end
+
+    def whole_form(action = "/", id = nil, html_class = nil, local: false, **options)
+      contents = block_given? ? yield : ""
+
+      method, multipart = options.values_at(:method, :multipart)
+
+      form_text(action, id, html_class, local, multipart, method) + hidden_fields(options.slice :method, :skip_enforcing_utf8) + contents + "</form>"
+    end
+
+    def protect_against_forgery?
+      false
+    end
+
+    def with_locale(testing_locale = :label)
+      old_locale, I18n.locale = I18n.locale, testing_locale
+      yield
+    ensure
+      I18n.locale = old_locale
+    end
+end