fields_for

Creates a scope around a specific model object like form_for, but doesn’t create the form tags themselves. This makes fields_for suitable for specifying additional model objects in the same form.

Although the usage and purpose of fields_for is similar to form_for's, its method signature is slightly different. Like form_for, it yields a FormBuilder object associated with a particular model object to a block, and within the block allows methods to be called on the builder to generate fields associated with the model object. Fields may reflect a model object in two ways - how they are named (hence how submitted values appear within the params hash in the controller) and what default values are shown when the form the fields appear in is first displayed. In order for both of these features to be specified independently, both an object name (represented by either a symbol or string) and the object itself can be passed to the method separately -

<%= form_for @person do |person_form| %>
  First name: <%= person_form.text_field :first_name %>
  Last name : <%= person_form.text_field :last_name %>

  <%= fields_for :permission, @person.permission do |permission_fields| %>
    Admin?  : <%= permission_fields.check_box :admin %>
  <% end %>

  <%= person_form.submit %>
<% end %>

In this case, the checkbox field will be represented by an HTML input tag with the name attribute permission[admin], and the submitted value will appear in the controller as params[:permission][:admin]. If @person.permission is an existing record with an attribute admin, the initial state of the checkbox when first displayed will reflect the value of @person.permission.admin.

Often this can be simplified by passing just the name of the model object to fields_for -

<%= fields_for :permission do |permission_fields| %>
  Admin?: <%= permission_fields.check_box :admin %>
<% end %>

…in which case, if :permission also happens to be the name of an instance variable @permission, the initial state of the input field will reflect the value of that variable’s attribute @permission.admin.

Alternatively, you can pass just the model object itself (if the first argument isn’t a string or symbol fields_for will realize that the name has been omitted) -

<%= fields_for @person.permission do |permission_fields| %>
  Admin?: <%= permission_fields.check_box :admin %>
<% end %>

and fields_for will derive the required name of the field from the class of the model object, e.g. if @person.permission, is of class Permission, the field will still be named permission[admin].

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

fields_for tries to be smart about parameters, but it can be confused if both name and value parameters are provided and the provided value has the shape of an option Hash. To remove the ambiguity, explicitly pass an option Hash, even if empty.

<%= form_for @person do |person_form| %>
  ...
  <%= fields_for :permission, @person.permission, {} do |permission_fields| %>
    Admin?: <%= check_box_tag permission_fields.field_name(:admin), @person.permission[:admin] %>
  <% end %>
  ...
<% end %>

Nested Attributes Examples

When the object belonging to the current scope has a nested attribute writer for a certain attribute, fields_for will yield a new scope for that attribute. This allows you to create forms that set or change the attributes of a parent object and its associations in one go.

Nested attribute writers are normal setter methods named after an association. The most common way of defining these writers is either with accepts_nested_attributes_for in a model definition or by defining a method with the proper name. For example: the attribute writer for the association :address is called address_attributes=.

Whether a one-to-one or one-to-many style form builder will be yielded depends on whether the normal reader method returns a single object or an array of objects.

One-to-one

Consider a Person class which returns a single Address from the address reader method and responds to the address_attributes= writer method:

class Person
  def address
    @address
  end

  def address_attributes=(attributes)
    # Process the attributes hash
  end
end

This model can now be used with a nested fields_for, like so:

<%= form_for @person do |person_form| %>
  ...
  <%= person_form.fields_for :address do |address_fields| %>
    Street  : <%= address_fields.text_field :street %>
    Zip code: <%= address_fields.text_field :zip_code %>
  <% end %>
  ...
<% end %>

When address is already an association on a Person you can use accepts_nested_attributes_for to define the writer method for you:

class Person < ActiveRecord::Base
  has_one :address
  accepts_nested_attributes_for :address
end

If you want to destroy the associated model through the form, you have to enable it first using the :allow_destroy option for accepts_nested_attributes_for:

class Person < ActiveRecord::Base
  has_one :address
  accepts_nested_attributes_for :address, allow_destroy: true
end

Now, when you use a form element with the _destroy parameter, with a value that evaluates to true, you will destroy the associated model (e.g. 1, ‘1’, true, or ‘true’):

<%= form_for @person do |person_form| %>
  ...
  <%= person_form.fields_for :address do |address_fields| %>
    ...
    Delete: <%= address_fields.check_box :_destroy %>
  <% end %>
  ...
<% end %>

One-to-many

Consider a Person class which returns an array of Project instances from the projects reader method and responds to the projects_attributes= writer method:

class Person
  def projects
    [@project1, @project2]
  end

  def projects_attributes=(attributes)
    # Process the attributes hash
  end
end

Note that the projects_attributes= writer method is in fact required for fields_for to correctly identify :projects as a collection, and the correct indices to be set in the form markup.

When projects is already an association on Person you can use accepts_nested_attributes_for to define the writer method for you:

class Person < ActiveRecord::Base
  has_many :projects
  accepts_nested_attributes_for :projects
end

This model can now be used with a nested fields_for. The block given to the nested fields_for call will be repeated for each instance in the collection:

<%= form_for @person do |person_form| %>
  ...
  <%= person_form.fields_for :projects do |project_fields| %>
    <% if project_fields.object.active? %>
      Name: <%= project_fields.text_field :name %>
    <% end %>
  <% end %>
  ...
<% end %>

It’s also possible to specify the instance to be used:

<%= form_for @person do |person_form| %>
  ...
  <% @person.projects.each do |project| %>
    <% if project.active? %>
      <%= person_form.fields_for :projects, project do |project_fields| %>
        Name: <%= project_fields.text_field :name %>
      <% end %>
    <% end %>
  <% end %>
  ...
<% end %>

Or a collection to be used:

<%= form_for @person do |person_form| %>
  ...
  <%= person_form.fields_for :projects, @active_projects do |project_fields| %>
    Name: <%= project_fields.text_field :name %>
  <% end %>
  ...
<% end %>

If you want to destroy any of the associated models through the form, you have to enable it first using the :allow_destroy option for accepts_nested_attributes_for:

class Person < ActiveRecord::Base
  has_many :projects
  accepts_nested_attributes_for :projects, allow_destroy: true
end

This will allow you to specify which models to destroy in the attributes hash by adding a form element for the _destroy parameter with a value that evaluates to true (e.g. 1, ‘1’, true, or ‘true’):

<%= form_for @person do |person_form| %>
  ...
  <%= person_form.fields_for :projects do |project_fields| %>
    Delete: <%= project_fields.check_box :_destroy %>
  <% end %>
  ...
<% end %>

When a collection is used you might want to know the index of each object in the array. For this purpose, the index method is available in the FormBuilder object.

<%= form_for @person do |person_form| %>
  ...
  <%= person_form.fields_for :projects do |project_fields| %>
    Project #<%= project_fields.index %>
    ...
  <% end %>
  ...
<% end %>

Note that fields_for will automatically generate a hidden field to store the ID of the record. There are circumstances where this hidden field is not needed and you can pass include_id: false to prevent fields_for from rendering it automatically.

# File actionview/lib/action_view/helpers/form_helper.rb, line 2284
      def fields_for(record_name, record_object = nil, fields_options = nil, &block)
        fields_options, record_object = record_object, nil if fields_options.nil? && record_object.is_a?(Hash) && record_object.extractable_options?
        fields_options ||= {}
        fields_options[:builder] ||= options[:builder]
        fields_options[:namespace] = options[:namespace]
        fields_options[:parent_builder] = self

        case record_name
        when String, Symbol
          if nested_attributes_association?(record_name)
            return fields_for_with_nested_attributes(record_name, record_object, fields_options, block)
          end
        else
          record_object = @template._object_for_form_builder(record_name)
          record_name   = model_name_from_record_or_class(record_object).param_key
        end

        object_name = @object_name
        index = if options.has_key?(:index)
          options[:index]
        elsif defined?(@auto_index)
          object_name = object_name.to_s.delete_suffix("[]")
          @auto_index
        end

        record_name = if index
          "#{object_name}[#{index}][#{record_name}]"
        elsif record_name.end_with?("[]")
          "#{object_name}[#{record_name[0..-3]}][#{record_object.id}]"
        else
          "#{object_name}[#{record_name}]"
        end
        fields_options[:child_index] = index

        @template.fields_for(record_name, record_object, fields_options, &block)
      end