fields_for(object_name, *args, &proc) public

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. Example:

  <% form_for :person, @person, :url => { :action => "update" } 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 %>
  <% end %>

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

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April 8, 2009 - (>= v2.3.2)
13 thanks

Setting child_index while using nested attributes mass assignment

When using nested attributes mass assignment sometimes you will want to add new records with javascript. You can do it with pure javascript, but if HTML is long your javascript will be long and messy and it will not be DRY as probably you already have a partial for it.

So to add a partial dynamically you can do something like that (notice string “index_to_replace_with_js”):


def add_object_link(name, form, object, partial, where)
  options = {:parent => true}.merge(options)
  html = render(:partial => partial, :locals => { :form => form}, :object => object)
  link_to_function name, %{
    var new_object_id = new Date().getTime() ;
    var html = jQuery(#{js html}.replace(/index_to_replace_with_js/g, new_object_id)).hide();

js method in one of helpers (from minus mor plugin)

def js(data)
  if data.respond_to? :to_json

This method will generate link adding generated partial to html.

The thing that is not mentioned in docs is how to set child_index. You must add it as an argument in hash.

Example of partial

<% form.fields_for :tasks, task, :child_index => 
       (task.new_record? ? "index_to_replace_with_js" : nil) do |tasks_form| %>
  <% tasks_form.text_field :name %>
<% end %>

Using add_object_link

<% form_for :project do |form| %>
  <div id="tasks">
    <%# displaying existing tasks %>

  <%= add_object_link("New task, form, Task.new, "task", "#tasks") %>

<% end %>

Thanks to child_index after insertion it will change indexes to current time in miliseconds so added tasks will have different names and ids.

January 22, 2009
5 thanks

Using fields_for with has_many associations

If you want to edit each element of an array of objects (such as with a has_many type association), you will need to include “[]” in your field parameter name, like so:

<% fields_for "object[]" do |subfield| -%>
 <% end -%>

Because you named the field parameter “object[]”, fields_for will assume you have an instance variable @object to use for the fields’ values. To fake this, you can do something like:

<% objects.each do |@object| -%>
  <% fields_for "object[]" do |subfield| -%>
  <% end -%>
<% end -%>

If that looks like sacrilegious Rails code to you, then you could consider:

<% objects.each do |object| -%>
  <% fields_for "object[]", object do |subfield| -%>
  <% end -%>
<% end -%>

In either case, params[:object] will be a hash where the ID of each object (determined via ActiveRecord::Base#to_param ) is associated with a hash of its new values:

params = { 'object' => { '123' => { 'field' => 'newval' }, '159' => { 'field' => 'newval' } } }
October 23, 2009
2 thanks

Setting child_index while using nested attributes mass assignment with prototype

First of all, drogus idea really helped me. I’m not using jQuery, therefore I implemented my own version:


def add_object_link(name, where, render_options)
  html = render(render_options)

  link_to_function name, %{
    Element.insert('#{where}', #{html.to_json}.replace(/index_to_replace_with_js/g, new Date().getTime()));

Using add_object_link

<%= add_object_link 'Add asset', 'assets', :partial => 'assets/asset', :object => Asset.new, :locals => { :f => f } %>
August 21, 2009
2 thanks

Passing parameters to custom formbuilders

If you implement your own formbuilder, the options passed are available as @options inside your formbuilder. If you want those configuration options passed to all builders in the fields_for sections, use the following code in your form builder:

def fields_for_with_options(record_or_name_or_array, *args, &block) options = args.extract_options! fields_for_without_options(record_or_name_or_array, *(args << options.merge(@options)), &block) end alias_method_chain :fields_for, :options


form_for @my_object, :builder => MyCustomFormbuilder, :some_setting => :cool
October 7, 2010
1 thank

Preserve order of elements within fields_for

I had the @colleagues collection prepared that I wanted to be rendered within fields_for block. However it was searchlogic object with filtering/sorting applied, and the sort order was not preserved in resultant view.

<%= f.fields_for :evaluators, @colleagues do |builder| %>
<%= render "colleague", :f => builder %>
<% end %>  

Solution to this problem was typecasting this collection to array

<%= f.fields_for :evaluators, @colleagues.to_a do |builder| %>
<%= render "colleague", :f => builder %>
<% end %>  
March 29, 2012 - (v2.3.2 - v3.2.1)
1 thank

JQuery script for dynamically adding and removing fields_for

I like drogus idea. But I wanted a cleaner one, so I created an unobtrusive JQuery script to have the same functionality.

Example Usage:

<%= form_for @post do |form| %>
  Title: <%= form.text_field :title %>
  Body: <%= form.text_field :body %>

  <div id="tag-list"></div>

  <div class="numerous">
    <div class="numerous-form">
      <%= form.fields_for :tag, Tag.new, :child_index => "replace_this" do |f| %>
        <%= f.text_field :name %>
        <%= f.hidden_field :_destroy, :value => 0, :class => "numerous-remove-field" %>
        <%= link_to "delete", "#", :class => "numerous-remove" %>
      <% end %>

    <%= link_to "add tag", "#", :class => "numerous-add", :id => "for-tag-list" %>
<% end %>

See script at: http://github.com/kbparagua/numerous.js

July 11, 2013
0 thanks

Using fields_for with collection (no association)

accepts_nested_attributes_for has some detractors: http://blog.codeclimate.com/blog/2012/10/17/7-ways-to-decompose-fat-activerecord-models

But fields_for is defined both on FormBuilder and FormHelper and is still useful even when accepts_nested_attributes_for is not being used. Consider a “to do list” application where we wish to edit and update a list of tasks (Task model). I like to use a TaskCollectionController to manage the actions on the collection, it feels more RESTful than overloading a TaskController:

class TaskCollectionController < ApplicationController
# task_collection_edit GET /task_collection/edit 
  def edit
    @tasks = Task.all

# task_collection_update PUT /task_collection/update
  def update
    # it would be better to use a TaskCollection form object here
    # to make this controller skinny,
    # for the sake of brevity I skipped the form object
    params[:tasks].values.each do |attrs|
      if attrs[:_destroy] == '1'
      elsif attrs[:id].blank?
    redirect_to task_collection_edit_path

In the edit view, I use fields_for to index added tasks, and it also will insert the id as a hidden field for existing tasks:

%h1 Edit Task Collection
= form_tag task_collection_update_path, :method => :put do
      %th Name
      %th Priority
    - @tasks.each do |task|
      = fields_for 'tasks[]', task, :hidden_field_id => true do |task_form|
        = render :partial => 'task_form', :locals => {:task_form => task_form}

  = button_tag 'Save'

= button_tag "Add task", :type => :button, :id => :add_task

%script{:type => 'html/template', :id => 'task_form_template'}
  = fields_for 'tasks[]', Task.new, :index => 'NEW_RECORD', :hidden_field_id => true do |task_form| render(:partial => 'task_form', :locals => {:task_form => task_form}); end

    task_form = function(){ return $('#task_form_template').text().replace(/NEW_RECORD/g, new Date().getTime())}
    var add_task = function(){ $('#tasks').append(task_form()) }
    $('#add_task').on('click', add_task)

:hidden_field_id => true triggers the insertion of the id field, and a placeholder index “NEW_RECORD” is replaced by javascript when a task is added, as others have described here. When there’s no association, the index key is :index, vs. :child_index in the case of an association.

Here’s the partial used in the edit view:

  %td= task_form.text_field :name
  %td= task_form.select :priority, [1,2,3,4,5]
  %td= remove_item(task_form)

The remove_item helper triggers the deletion of persisted tasks:

module TaskCollectionHelper
  def remove_item(form_builder)
    if form_builder.object.new_record?
      # If the task is a new record, remove the div from the dom
      link_to_function( 'Remove', "$(this).closest('tr').remove()")
      # However if it's a "real" record it has to be deleted from the database,
      # hide the form and mark for destruction
      form_builder.hidden_field(:_destroy) +
      link_to_function( 'Remove', "$(this).closest('tr').hide(); $(this).siblings().attr('value',1)")