Rails accepts_nested_attributes_for with f.fields_for and AJAX

It Is Possible

There’s a very, very good tutorial on this here: http://pikender.in/2013/04/20/child-forms-using-fields_for-through-ajax-rails-way/

We also recently implemented this type of form on one of our development apps. If you goto & http://emailsystem.herokuapp.com, sign up (free) and click “New Message”. The “Subscribers” part uses this technology

BTW we did this manually. Cocoon actually looks really good, and seems to use the same principles as us. There’s also a RailsCast, but this only works for single additions (I think)

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f.fields_for

The way you do it is to use a series of partials which dynamically build the fields you need. From your code, it looks like you have the fundamentals in place (the form is working), so now it’s a case of building several components to handle the AJAX request:

1. You need to handle the AJAX on the controller (route + controller action)
2. You need to put your f.fields_for into partials (so they can be called with Ajax)
3. You need to handle the build functionality in the model

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Handling AJAX With The Controller

Firstly, you need to handle the Ajax requests in the controller

To do this, you need to add a new “endpoint” to the routes. This is ours:

 resources :messages, :except => [:index, :destroy] do
    collection do
       get :add_subscriber
    end
 end

The controller action then translates into:

#app/controllers/messages_controller.rb
#Ajax Add Subscriber
def add_subscriber
    @message = Message.build
    render "add_subscriber", :layout => false
end

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Add Your f.fields_for Into Partials

To handle this, you need to put your f.fields_for into partials. Here is the code form our form:

#app/views/resources/_message_subscriber_fields.html.erb
<%= f.fields_for :message_subscribers, :child_index => child_index do |subscriber| %>
    <%= subscriber.collection_select(:subscriber_id, Subscriber.where(:user_id => current_user.id), :id, :name_with_email, include_blank: 'Subscribers') %>
<% end %>

#app/views/messages/add_subscriber.html.erb
<%= form_for @message, :url => messages_path, :authenticity_token => false do |f| %>
        <%= render :partial => "resources/message_subscriber_fields", locals: {f: f, child_index: Time.now.to_i} %>
<% end %>

#app/views/messages/new.html.erb
<% child_index = Time.now.to_i %>
<div id="subscribers">
    <div class="title">Subscribers</div>
    <%= render :partial => "message_subscriber_fields", locals: {f: f, child_index: child_index } %>
</div>

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Extend Your Build Functionality To Your Model

To keep things dry, we just created a build function in the model, which we can call each time:

   #Build
    def self.build
       message = self.new
       message.message_subscribers.build
       message
    end

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Child_Index

Your best friend here is child_index

If you’re adding multiple fields, the big problem you’ll have is incrementing the [id] of the field (this was the flaw we found with Ryan Bates’ tutorial)

The way the first tutorial I posted solved this was to just set the child_index of the new fields with Time.now.to_i. This sets a unique id, and because the actual ID of the new field is irrelevant, you’ll be able to add as many fields as you like with it

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JQuery

    #Add Subscriber
    $ ->
      $(document).on "click", "#add_subscriber", (e) ->
         e.preventDefault();

         #Ajax
         $.ajax
           url: '/messages/add_subscriber'
           success: (data) ->
               el_to_add = $(data).html()
               $('#subscribers').append(el_to_add)
           error: (data) ->
               alert "Sorry, There Was An Error!"