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).

1 files changed, 2134 insertions, 0 deletions

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