How to Create Nested Forms in Rails 4

Question

I'm trying to let a user create Exercises with Equipment and Muscles in many-to-many relationships through their respective join tables( exercise_equipment, exercise_muscles ). I've gotten the form working for adding one equipment/muscle per exercise, but cannot figure out how to add a link to add another field to the form on the fly.

I've checked out RailsCasts, this post, have asked it as a side question on a previous post of my own, but simply cannot get this functionality to work. I'm fairly new to Rails 4 and am still trying to learn Javascript, but I'd love a thorough explanation of how to set this up the Rails 4 way!

My Models:

# id        :integer
# name      :string
# is_public :boolean
Exercise
    has_many :exercise_equipment
    has_many :equipment, :through => :exercise_equipment
    accepts_nested_attributes_for :exercise_equipment


# id           :integer
# exercise_id  :integer
# equipment_id :integer
# optional     :boolean
ExerciseEquipment
    belongs_to :exercise
    belongs_to :equipment
    accepts_nested_attributes_for :equipment


# id   :integer
# name :string
Equipment
    has_many :exercise_equipment
    has_many :exercises, :through => :exercise_equipment

My Controller Methods:

def new
  @exercise = Exercise.new
  @exercise.exercise_equipment.build
  @exercise.exercise_muscles.build
end

def create
  exercise = current_user.exercises.new( exercise_params )
  if exercise.save!
    redirect_to exercise
  else
    render 'new'
  end
end

views/exercises/new.html.erb

<h1>Create New Exercise</h1>

<%= form_for @exercise do |f| %>
  <%= render 'form', f: f %>
  <%= f.submit "New Exercise" %>
<% end %>

views/exercises/_form.html.erb

<%= f.label :name %><br />
<%= f.text_field :name, autofocus: true %>

<%= f.check_box :is_public %> Public

<%= f.fields_for :exercise_muscles do |emf| %>
  <%= emf.collection_select :muscle_id, Muscle.all, :id, :name, { include_hidden: false } %>
<% end %>

<%= f.fields_for :exercise_equipment do |eef| %>
  <%= eef.collection_select :equipment_id, Equipment.all, :id, :name, { include_hidden: false } %>
  <%= eef.check_box :optional %> Optional
<% end %>

Answer 1

Ajax

Cannot figure out how to add a link to add another field to the form on the fly

The "Rails way" of doing that is to "pull" a new instance of the fields_for block from an ajax request.

The reason why Ajax is recommended is because it's the "Rails way" to do it - completely modular & extensible:

#config/routes.rb
resources :exercises do
   get :add_field, on: :collection
end

#app/models/exercise.rb
Class Exercise < ActiveRecord::Base
   ...
   def self.build #-> allows you to call a single method
      exercise = self.new
      exercise.exercise_equipment.build
      exercise.exercise_muscles.build
      return
   end
end

#app/controllers/exercises_controller.rb
Class ExercisesController < ApplicationController
   def add_field
      @exercise = Exercise.build
      respond_to do |format|
         format.html
         format.js
      end
   end
end

#app/views/exercises/new.html.erb
<%= form_for @exercise do |f| %>
   <%= render "fields", locals: { f: f } %>
   <%= f.submit %>
<% end %>

#app/views/exercises/add_field.js.erb
$("#form_element").append("<%=j render "exercises/form", locals: { exercise: @exercise } %>");

#app/views/exercises/_form.html.erb
<%= form_for exercise do |f| %>
   <%= render "fields", locals: { f: f } %>
<% end %>

#app/views/exercises/_fields.html.erb
<%= f.fields_for :exercise_muscles, child_index: Time.now.to_i do |emf| %>
    <%= emf.collection_select :muscle_id, Muscle.all, :id, :name, { include_hidden: false } %>
<% end %>
<%= link_to "New Field", exercises_add_fields_path, remote: :true %>

<%= f.fields_for :exercise_equipment, child_index: Time.now.to_i do |eef| %>
   <%= eef.collection_select :equipment_id, Equipment.all, :id, :name, { include_hidden: false } %>
   <%= eef.check_box :optional %> Optional
<% end %>
<%= link_to "New Field", exercises_add_fields_path, remote: :true %>

This will give you the ability to create the fields through an ajax call; which is the correct way to do it

Answer 2

After attempting to use Rich's solution, I wanted to find one that was a bit more minimal in terms of the code used. I found the gem Cocoon, which works great and was very simple to integrate.

My main _form view:

<div class="form-group">
  <%= f.label :name %><br />
  <%= f.text_field :name, autofocus: true %>
</div>

<div class="form-group">
  <%= f.check_box :is_public %> Public
</div>

<div class="form-group">
  <%= f.fields_for :exercise_muscles do |emf| %>
    <%= render 'exercise_muscle_fields', :f => emf %>
  <% end %>
  <%= link_to_add_association 'Add Muscle', f, :exercise_muscles %>
</div>

<div class="form-group">
  <%= f.fields_for :exercise_equipment do |eef| %>
    <%= render 'exercise_equipment_fields', :f => eef %>
  <% end %>
  <%= link_to_add_association 'Add Equipment', f, :exercise_equipment %>
</div>

As can be seen here, the addition of a simple "link_to_add_association" method takes care of all of the Javascript in the background. For future readers, here are the partials that each of these form-groups contain:

_exercise_muscle_fields:

<%= f.collection_select :muscle_id, Muscle.all.order( 'muscle_group_id ASC' ), :id, :name, { include_hidden: false } %>

_exercise_equipment_fields:

<%= f.collection_select :equipment_id, Equipment.all.order( 'name ASC' ), :id, :name, { include_hidden: false } %>
<%= f.check_box :optional %> Optional