Flowdock
form_for(record_or_name_or_array, *args, &proc) public

Creates a form and a scope around a specific model object that is used as a base for questioning about values for the fields.

  <% form_for :person, @person, :url => { :action => "update" } do |f| %>
    First name: <%= f.text_field :first_name %>
    Last name : <%= f.text_field :last_name %>
    Biography : <%= f.text_area :biography %>
    Admin?    : <%= f.check_box :admin %>
  <% end %>

Worth noting is that the form_for tag is called in a ERb evaluation block, not an ERb output block. So that’s <% %>, not <%= %>. Also worth noting is that form_for yields a form_builder object, in this example as f, which emulates the API for the stand-alone FormHelper methods, but without the object name. So instead of text_field :person, :name, you get away with f.text_field :name.

Even further, the form_for method allows you to more easily escape the instance variable convention. So while the stand-alone approach would require text_field :person, :name, :object => person to work with local variables instead of instance ones, the form_for calls remain the same. You simply declare once with :person, person and all subsequent field calls save :person and :object => person.

Also note that form_for doesn’t create an exclusive scope. It’s still possible to use both the stand-alone FormHelper methods and methods from FormTagHelper. For example:

  <% form_for :person, @person, :url => { :action => "update" } do |f| %>
    First name: <%= f.text_field :first_name %>
    Last name : <%= f.text_field :last_name %>
    Biography : <%= text_area :person, :biography %>
    Admin?    : <%= check_box_tag "person[admin]", @person.company.admin? %>
  <% end %>

Note: This also works for the methods in FormOptionHelper and DateHelper that are designed to work with an object as base, like FormOptionHelper#collection_select and DateHelper#datetime_select.

HTML attributes for the form tag can be given as :html => {…}. For example:

  <% form_for :person, @person, :html => {:id => 'person_form'} do |f| %>
    ...
  <% end %>

The above form will then have the id attribute with the value </tt>person_form</tt>, which you can then style with CSS or manipulate with JavaScript.

Relying on record identification

In addition to manually configuring the form_for call, you can also rely on record identification, which will use the conventions and named routes of that approach. Examples:

  <% form_for(@post) do |f| %>
    ...
  <% end %>

This will expand to be the same as:

  <% form_for :post, @post, :url => post_path(@post), :html => { :method => :put, :class => "edit_post", :id => "edit_post_45" } do |f| %>
    ...
  <% end %>

And for new records:

  <% form_for(Post.new) do |f| %>
    ...
  <% end %>

This will expand to be the same as:

  <% form_for :post, @post, :url => posts_path, :html => { :class => "new_post", :id => "new_post" } do |f| %>
    ...
  <% end %>

You can also overwrite the individual conventions, like this:

  <% form_for(@post, :url => super_post_path(@post)) do |f| %>
    ...
  <% end %>

And for namespaced routes, like admin_post_url:

  <% form_for([:admin, @post]) do |f| %>
   ...
  <% end %>

Customized form builders

You can also build forms using a customized FormBuilder class. Subclass <a href="/rails/ActionView/Helpers/FormBuilder">FormBuilder</a> and override or define some more helpers, then use your custom builder. For example, let’s say you made a helper to automatically add labels to form inputs.

  <% form_for :person, @person, :url => { :action => "update" }, :builder => LabellingFormBuilder do |f| %>
    <%= f.text_field :first_name %>
    <%= f.text_field :last_name %>
    <%= text_area :person, :biography %>
    <%= check_box_tag "person[admin]", @person.company.admin? %>
  <% end %>

In many cases you will want to wrap the above in another helper, so you could do something like the following:

  def labelled_form_for(record_or_name_or_array, *args, &proc)
    options = args.extract_options!
    form_for(record_or_name_or_array, *(args << options.merge(:builder => LabellingFormBuilder)), &proc)
  end

If you don’t need to attach a form to a model instance, then check out FormTagHelper#form_tag.

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July 22, 2008
18 thanks

Nested resources in form_for

If you like doing things RESTfully and have a model relationship like:

Post_ has many Comments_

Then you can construct a form_for within your view to mirror this relationship when creating comments:

form_for [@post, @comment] do |f|
  ...
end

You also need to make sure your routes reflect this relationship:

map.resources :post, :has_many => [:comments]
August 5, 2008
17 thanks

Multipart form

Don’t forget to add :multipart => true if you have file upload in your form.

<% form_for "user", :html => { :multipart => true } do |f| %>
January 27, 2009 - (>= v2.2.1)
5 thanks

Getting the object in a partial

If you need to get the object for the form inside a partial, and can’t use the instance variable, use the #object method… This is particularly useful when you’re dealing with single-table inheritance subclasses (e.g. MyOtherClass inherits from MyClass) or when you are using the same partial across different controllers.

new.html.erb

<% form_for(@my_object) do %>
  <%= render :partial => 'form' %>
  <%= submit_tag 'Create' %>
<% end %>

_form.html.erb

<% if f.object.class.is_a? MyClass %>
 <%# do something... %>
<% elsif f.object.is_a? MyOtherClass %>
  <%# do something else... %>
<% end %>
October 8, 2008
3 thanks

Seriously! Do not forget the brackets

thank you source jamesandre.ws

the form_for([:admin, @user]) must have the [] brackets to avoid errors like “Only get requests are allowed”

<% form_for([:admin, @user]) do |f| %>
<%= render :partial => 'form' %>
<%= submit_tag "Create" %>
<% end %>
September 28, 2008 - (v2.0.0 - v2.1.0)
3 thanks

has_one Nesting in Rails 2.0

Routers:

map.resources :user, :has_one => [:avatar]

Views:

form_for [@user, @avatar], :url => user_avatar_url(@user) do |f|
...
end
September 25, 2008 - (v2.1.0)
3 thanks

Compare old and new form for

Old form for

<% form_for :user, :url => users_path do %>
  <%= render :partial => 'form' %>
  <%= submit_tag 'Create' %>
<% end %>

New form for

<% form_for(@user) do |f| %>
  <%= render :partial => f %>
  <%= submit_tag 'Create' %>
<% end %>
November 2, 2008
2 thanks

params hash gets the model id automatically

The params hash gets automatically populated with the id of every model that gets passed to form_for. If we were creating a song inside an existing album:

URL:/albums/209/songs/new
form_for [@album, @song] do |f| 
  ...
  f.submit "Add"
end

The params hash would be:

params = {"commit"=>"Add", 
          "authenticity_token"=>"...",
          "album_id"=>"209",
          "song"=>{"song_attributes"=>{...}}
          }

So, in the songs_controller you could use this album_id in a before_filter:

before_filter :find_album
protected
def find_album
  @album = Album.find(params[:album_id])
end

If you only did this:

form_for @song do |f| 

You would get this params hash:

params = {"commit"=>"Add", 
          "authenticity_token"=>"...",
          "song"=>{"song_attributes"=>{...}}
          }  
April 9, 2010
2 thanks

The :method goes in the :html option

When using a restful form helper and you want to use a method other than POST, remember to put the :method in the :html option.

e.g. To send a DELETE request instead of the usual POST (with a nested resource thrown in for good measure) use:

<% form_for [@post, @comment], :html => { :method => :delete } do |f| -%>
October 26, 2011
2 thanks

Adding to the URL

If you want to use polymorphic routing for your object but you also need to specify other stuff like an anchor, you can explicitly generate the polymorphic url with extra options:

form_for @candidate,
  :url => polymorphic_path(@candidate, :anchor => 'signup')
November 4, 2009
2 thanks

Using hidden tags

To use an <input type=“hidden” /> tag, use the following syntax:

<% form_for(@post) do |f| %>
  <%= f.hidden_field :user_id, { :value => user.id } %>
<% end %>
June 22, 2011
1 thank

:url conflicts with :format

If you are passing both :url and :format, url overwrites the use of format, so you’ll need to pass it in the url like so:

form_for user, :url => user_path(@user, :format => :json)
July 23, 2014 - (v3.2.1)
0 thanks

form_for with namescope and polymorphic path

<%= form_for([:namescope_name, @object], :url => polymorphic_path([:namescope_name, @objectable, @object])) do |form| %>


for the routes.

namescope :admin do

resources :articles do
  resources :comments
end
resources :photos do
  resources :comments
end

end

<%= form_for([:admin, @comment], :url => polymorphic_path([:admin, @commentable, @comment])) do |form| %>

Note : @commentable = find_commentable

June 24, 2010
0 thanks

authenticity_token

<div style=“margin:0;padding:0”>

<input name="authenticity_token" type="hidden" value="f755bb0ed134b76c432144748a6d4b7a7ddf2b71" /> 

</div>

Helper generates a div element with a hidden input inside. This 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”.

January 16, 2012
0 thanks

form_for with :as routing

The following will not work if your post model is routed with the :as option:

form_for(@post)

Instead, use the helper with your custom name:

form_for(@post, :url => edit_renamedpost_path(@post))
June 25, 2012 - (<= v2.3.8)
0 thanks

Adding Additional Parameters to form_for

If you want to add additional parameters to the form_for helper, but still want to use one form for both your “create” and your “update” actions, you can add the additional parameters to the :url option, but you need to omit the :controller and :action keys.

form_for(@user, :url => {:param1 => "value1", :param2 => "value2"}) do |f|

or

form_for([@post, @comment], :url => {:param1 => "value1", :param2 => "value2"}) do |f| 

where param1 and param2 are not :controller or :action

March 7, 2014
0 thanks

form_for with :path route

Similar to danwich’s note, if you specify a route using the :path option

resource :posts, path: 'articles'

then the form_for tag must specify the :url option

form_for(@post), url: post_path(@post)