h2. Rails Form helpers
Forms in web applications are an essential interface for user input. However, form markup can quickly become tedious to write and maintain because of form control naming and their numerous attributes. Rails deals away with these complexities by providing view helpers for generating form markup. However, since they have different use-cases, developers are required to know all the differences between similar helper methods before putting them to use.
In this guide you will:
* Create search forms and similar kind of generic forms not representing any specific model in your application
* Make model-centric forms for creation and editing of specific database records
* Generate select boxes from multiple types of data
* Understand the date and time helpers Rails provides
* Learn what makes a file upload form different
* Learn some cases of building forms to external resources
* Find out where to look for complex forms
endprologue.
NOTE: This guide is not intended to be a complete documentation of available form helpers and their arguments. Please visit "the Rails API documentation":http://api.rubyonrails.org/ for a complete reference.
h3. Dealing with Basic Forms
The most basic form helper is +form_tag+.
<%= form_tag do %>
Form contents
<% end %>
When called without arguments like this, it creates a form element that has the current page as its action and "post" as its method (some line breaks added for readability):
Sample output from +form_tag+:
If you carefully observe this output, you can see that the helper generated something you didn't specify: a +div+ element with two hidden input elements inside. The first input element with name +utf8+ enforces browsers to properly respect your form's character encoding and is generated for all forms whether action is "get" or "post". Second input element with name +authenticity_token+ is a security feature of Rails called *cross-site request forgery protection* and form helpers generate it for every form whose action is not "get" (provided that this security feature is enabled). You can read more about this in the "Ruby On Rails Security Guide":./security.html#_cross_site_reference_forgery_csrf.
NOTE: Throughout this guide, this +div+ with the hidden input elements will be stripped away to have clearer code samples.
h4. A Generic Search Form
Probably the most minimal form often seen on the web is a search form with a single text input for search terms. This form consists of:
# a form element with "GET" method,
# a label for the input,
# a text input element, and
# a submit element.
IMPORTANT: Always use "GET" as the method for search forms. This allows users to bookmark a specific search and get back to it. More generally Rails encourages you to use the right HTTP verb for an action.
To create this form you will use +form_tag+, +label_tag+, +text_field_tag+, and +submit_tag+, respectively.
A basic search form
<%= form_tag(search_path, :method => "get") do %>
<%= label_tag(:q, "Search for:") %>
<%= text_field_tag(:q) %>
<%= submit_tag("Search") %>
<% end %>
TIP: +search_path+ can be a named route specified in "routes.rb" as: match "search" => "search" This declares that path "/search" will be handled by action "search" belonging to controller "search".
The above view code will result in the following markup:
Besides +text_field_tag+ and +submit_tag+, there is a similar helper for _every_ form control in HTML.
TIP: For every form input, an ID attribute is generated from its name ("q" in the example). These IDs can be very useful for CSS styling or manipulation of form controls with JavaScript.
h4. Multiple Hashes in Form Helper Calls
By now you've seen that the +form_tag+ helper accepts 2 arguments: the path for the action and an options hash. This hash specifies the method of form submission and HTML options such as the form element's class.
As with the +link_to+ helper, the path argument doesn't have to be given a string. It can be a hash of URL parameters that Rails' routing mechanism will turn into a valid URL. However, this is a bad way to pass multiple hashes as method arguments:
form_tag(:controller => "people", :action => "search", :method => "get", :class => "nifty_form")
# =>
Here you wanted to pass two hashes, but the Ruby interpreter sees only one hash, so Rails will construct a URL with extraneous parameters. The correct way of passing multiple hashes as arguments is to delimit the first hash (or both hashes) with curly brackets:
form_tag({:controller => "people", :action => "search"}, :method => "get", :class => "nifty_form")
# =>
This is a common pitfall when using form helpers, since many of them accept multiple hashes. So in future, if a helper produces unexpected output, make sure that you have delimited the hash parameters properly.
WARNING: Do not delimit the second hash without doing so with the first hash, otherwise your method invocation will result in an +expecting tASSOC+ syntax error.
h4. Helpers for Generating Form Elements
Rails provides a series of helpers for generating form elements such as checkboxes, text fields and radio buttons. These basic helpers, with names ending in _tag such as +text_field_tag+ and +check_box_tag+ generate just a single +<input>+ element. The first parameter to these is always the name of the input. In the controller this name will be the key in the +params+ hash used to get the value entered by the user. For example, if the form contains
<%= text_field_tag(:query) %>
then the controller code should use
params[:query]
to retrieve the value entered by the user. When naming inputs, be aware that Rails uses certain conventions that control whether values are at the top level of the +params+ hash, inside an array or a nested hash and so on. You can read more about them in the parameter_names section. For details on the precise usage of these helpers, please refer to the "API documentation":http://api.rubyonrails.org/classes/ActionView/Helpers/FormTagHelper.html.
h5. Checkboxes
Checkboxes are form controls that give the user a set of options they can enable or disable:
<%= check_box_tag(:pet_dog) %>
<%= label_tag(:pet_dog, "I own a dog") %>
<%= check_box_tag(:pet_cat) %>
<%= label_tag(:pet_cat, "I own a cat") %>
output:
The second parameter to +check_box_tag+ is the value of the input. This is the value that will be submitted by the browser if the checkbox is ticked (i.e. the value that will be present in the +params+ hash). With the above form you would check the value of +params[:pet_dog]+ and +params[:pet_cat]+ to see which pets the user owns.
h5. Radio Buttons
Radio buttons, while similar to checkboxes, are controls that specify a set of options in which they are mutually exclusive (i.e. the user can only pick one):
<%= radio_button_tag(:age, "child") %>
<%= label_tag(:age_child, "I am younger than 21") %>
<%= radio_button_tag(:age, "adult") %>
<%= label_tag(:age_adult, "I'm over 21") %>
output:
As with +check_box_tag+ the second parameter to +radio_button_tag+ is the value of the input. Because these two radio buttons share the same name (age) the user will only be able to select one and +params[:age]+ will contain either "child" or "adult".
IMPORTANT: Always use labels for each checkbox and radio button. They associate text with a specific option and provide a larger clickable region.
h4. Other Helpers of Interest
Other form controls worth mentioning are the text area, password input and hidden input:
<%= text_area_tag(:message, "Hi, nice site", :size => "24x6") %>
<%= password_field_tag(:password) %>
<%= hidden_field_tag(:parent_id, "5") %>
output:
Hidden inputs are not shown to the user, but they hold data like any textual input. Values inside them can be changed with JavaScript.
TIP: If you're using password input fields (for any purpose), you might want to configure your application to prevent those parameters from being logged.
h3. Dealing with Model Objects
h4. Model Object Helpers
A particularly common task for a form is editing or creating a model object. While the +*_tag+ helpers can certainly be used for this task they are somewhat verbose as for each tag you would have to ensure the correct parameter name is used and set the default value of the input appropriately. Rails provides helpers tailored to this task. These helpers lack the _tag suffix, for example +text_field+, +text_area+.
For these helpers the first argument is the name of an instance variable and the second is the name of a method (usually an attribute) to call on that object. Rails will set the value of the input control to the return value of that method for the object and set an appropriate input name. If your controller has defined +@person+ and that person's name is Henry then a form containing:
<%= text_field(:person, :name) %>
will produce output similar to
Upon form submission the value entered by the user will be stored in +params[:person][:name]+. The +params[:person]+ hash is suitable for passing to +Person.new+ or, if +@person+ is an instance of Person, +@person.update_attributes+. While the name of an attribute is the most common second parameter to these helpers this is not compulsory. In the example above, as long as person objects have a +name+ and a +name=+ method Rails will be happy.
WARNING: You must pass the name of an instance variable, i.e. +:person+ or +"person"+, not an actual instance of your model object.
Rails provides helpers for displaying the validation errors associated with a model object. These are covered in detail by the "Active Record Validations and Callbacks":./active_record_validations_callbacks.html#displaying-validation-errors-in-the-view guide.
h4. Binding a Form to an Object
While this is an increase in comfort it is far from perfect. If Person has many attributes to edit then we would be repeating the name of the edited object many times. What we want to do is somehow bind a form to a model object, which is exactly what +form_for+ does.
Assume we have a controller for dealing with articles +app/controllers/articles_controller.rb+:
def new
@article = Article.new
end
The corresponding view +app/views/articles/new.html.erb+ using +form_for+ looks like this:
<%= form_for @article, :url => { :action => "create" }, :html => {:class => "nifty_form"} do |f| %>
<%= f.text_field :title %>
<%= f.text_area :body, :size => "60x12" %>
<%= f.submit "Create" %>
<% end %>
There are a few things to note here:
# +@article+ is the actual object being edited.
# There is a single hash of options. Routing options are passed in the +:url+ hash, HTML options are passed in the +:html+ hash.
# The +form_for+ method yields a *form builder* object (the +f+ variable).
# Methods to create form controls are called *on* the form builder object +f+
The resulting HTML is:
The name passed to +form_for+ controls the key used in +params+ to access the form's values. Here the name is +article+ and so all the inputs have names of the form +article[attribute_name]+. Accordingly, in the +create+ action +params[:article]+ will be a hash with keys +:title+ and +:body+. You can read more about the significance of input names in the parameter_names section.
The helper methods called on the form builder are identical to the model object helpers except that it is not necessary to specify which object is being edited since this is already managed by the form builder.
You can create a similar binding without actually creating +<form>+ tags with the +fields_for+ helper. This is useful for editing additional model objects with the same form. For example if you had a Person model with an associated ContactDetail model you could create a form for creating both like so:
<%= form_for @person, :url => { :action => "create" } do |person_form| %>
<%= person_form.text_field :name %>
<%= fields_for @person.contact_detail do |contact_details_form| %>
<%= contact_details_form.text_field :phone_number %>
<% end %>
<% end %>
which produces the following output:
The object yielded by +fields_for+ is a form builder like the one yielded by +form_for+ (in fact +form_for+ calls +fields_for+ internally).
h4. Relying on Record Identification
The Article model is directly available to users of the application, so -- following the best practices for developing with Rails -- you should declare it *a resource*:
resources :articles
TIP: Declaring a resource has a number of side-affects. See "Rails Routing From the Outside In":routing.html#resource-routing-the-rails-default for more information on setting up and using resources.
When dealing with RESTful resources, calls to +form_for+ can get significantly easier if you rely on *record identification*. In short, you can just pass the model instance and have Rails figure out model name and the rest:
## Creating a new article
# long-style:
form_for(@article, :url => articles_path)
# same thing, short-style (record identification gets used):
form_for(@article)
## Editing an existing article
# long-style:
form_for(@article, :url => article_path(@article), :html => { :method => "put" })
# short-style:
form_for(@article)
Notice how the short-style +form_for+ invocation is conveniently the same, regardless of the record being new or existing. Record identification is smart enough to figure out if the record is new by asking +record.new_record?+. It also selects the correct path to submit to and the name based on the class of the object.
Rails will also automatically set the +class+ and +id+ of the form appropriately: a form creating an article would have +id+ and +class+ +new_article+. If you were editing the article with id 23, the +class+ would be set to +edit_article+ and the id to +edit_article_23+. These attributes will be omitted for brevity in the rest of this guide.
WARNING: When you're using STI (single-table inheritance) with your models, you can't rely on record identification on a subclass if only their parent class is declared a resource. You will have to specify the model name, +:url+, and +:method+ explicitly.
h5. Dealing with Namespaces
If you have created namespaced routes, +form_for+ has a nifty shorthand for that too. If your application has an admin namespace then
form_for [:admin, @article]
will create a form that submits to the articles controller inside the admin namespace (submitting to +admin_article_path(@article)+ in the case of an update). If you have several levels of namespacing then the syntax is similar:
form_for [:admin, :management, @article]
For more information on Rails' routing system and the associated conventions, please see the "routing guide":routing.html.
h4. How do forms with PUT or DELETE methods work?
The Rails framework encourages RESTful design of your applications, which means you'll be making a lot of "PUT" and "DELETE" requests (besides "GET" and "POST"). However, most browsers _don't support_ methods other than "GET" and "POST" when it comes to submitting forms.
Rails works around this issue by emulating other methods over POST with a hidden input named +"_method"+, which is set to reflect the desired method:
form_tag(search_path, :method => "put")
output:
This will result in a +params+ hash that looks like
{'person' => {'name' => 'Bob', 'address' => {'23' => {'city' => 'Paris'}, '45' => {'city' => 'London'}}}}
Rails knows that all these inputs should be part of the person hash because you called +fields_for+ on the first form builder. By specifying an +:index+ option you're telling Rails that instead of naming the inputs +person[address][city]+ it should insert that index surrounded by [] between the address and the city. If you pass an Active Record object as we did then Rails will call +to_param+ on it, which by default returns the database id. This is often useful as it is then easy to locate which Address record should be modified. You can pass numbers with some other significance, strings or even +nil+ (which will result in an array parameter being created).
To create more intricate nestings, you can specify the first part of the input name (+person[address]+ in the previous example) explicitly, for example
<%= fields_for 'person[address][primary]', address, :index => address do |address_form| %>
<%= address_form.text_field :city %>
<% end %>
will create inputs like
As a general rule the final input name is the concatenation of the name given to +fields_for+/+form_for+, the index value and the name of the attribute. You can also pass an +:index+ option directly to helpers such as +text_field+, but it is usually less repetitive to specify this at the form builder level rather than on individual input controls.
As a shortcut you can append [] to the name and omit the +:index+ option. This is the same as specifying +:index => address+ so
<%= fields_for 'person[address][primary][]', address do |address_form| %>
<%= address_form.text_field :city %>
<% end %>
produces exactly the same output as the previous example.
h3. Forms to external resources
If you need to post some data to an external resource it is still great to build your from using rails form helpers. But sometimes you need to set an +authenticity_token+ for this resource. You can do it by passing an +:authenticity_token => 'your_external_token'+ parameter to the +form_tag+ options:
<%= form_tag 'http://farfar.away/form', :authenticity_token => 'external_token') do %>
Form contents
<% end %>
Sometimes when you submit data to an external resource, like payment gateway, fields you can use in your form are limited by an external API. So you may want not to generate an +authenticity_token+ hidden field at all. For doing this just pass +false+ to the +:authenticity_token+ option:
<%= form_tag 'http://farfar.away/form', :authenticity_token => false) do %>
Form contents
<% end %>
The same technique is available for the +form_for+ too:
<%= form_for @invoice, :url => external_url, :authenticity_token => 'external_token' do |f|
Form contents
<% end %>
Or if you don't want to render an +authenticity_token+ field:
<%= form_for @invoice, :url => external_url, :authenticity_token => false do |f|
Form contents
<% end %>
h3. Building Complex Forms
Many apps grow beyond simple forms editing a single object. For example when creating a Person you might want to allow the user to (on the same form) create multiple address records (home, work, etc.). When later editing that person the user should be able to add, remove or amend addresses as necessary. While this guide has shown you all the pieces necessary to handle this, Rails does not yet have a standard end-to-end way of accomplishing this, but many have come up with viable approaches. These include:
* As of Rails 2.3, Rails includes "Nested Attributes":./2_3_release_notes.html#nested-attributes and "Nested Object Forms":./2_3_release_notes.html#nested-object-forms
* Ryan Bates' series of Railscasts on "complex forms":http://railscasts.com/episodes/75
* Handle Multiple Models in One Form from "Advanced Rails Recipes":http://media.pragprog.com/titles/fr_arr/multiple_models_one_form.pdf
* Eloy Duran's "complex-forms-examples":https://github.com/alloy/complex-form-examples/ application
* Lance Ivy's "nested_assignment":https://github.com/cainlevy/nested_assignment/tree/master plugin and "sample application":https://github.com/cainlevy/complex-form-examples/tree/cainlevy
* James Golick's "attribute_fu":https://github.com/jamesgolick/attribute_fu plugin
h3. Changelog
* February 5, 2011: Added 'Forms to external resources' section. Timothy N. Tsvetkov
* April 6, 2010: Fixed document to validate XHTML 1.0 Strict. "Jaime Iniesta":http://jaimeiniesta.com
h3. Authors
* Mislav Marohnić
* "Frederick Cheung":credits.html#fcheung