Ruby on Rails: Saving joined attribute with nested_form populates the database with 'nil'

Question

I am currently creating a WebApp using RoR 4 and I am using has_many, though: associations between my databases.

I have 3 models Users, UsersSubject and Subjects given below:

class UsersSubject < ActiveRecord::Base
  belongs_to :users
  belongs_to :subjects
end

class Subject < ActiveRecord::Base
  has_many :users_subjects
  has_many :users, through: :users_subjects
end

class User < ActiveRecord::Base
  has_many :users_subjects, :class_name => 'UsersSubject'
  has_many :subjects, through: :users_subjects

  accepts_nested_attributes_for :subjects
end

I am trying to populate the UsersSubject database when I am updating the User using the User controller. Here is my from the partial form of the User:

<div class="control-group nested-fields">
  <div class="contols">
    <%= f.fields_for :subject do |subject| %>
      <%= subject.label "Subjects" %></br>
      <%= subject.collection_select(:subject_id, Subject.all, :id, :name) %>
    <% end %>
  </div>
</div>

and here is my controller:

def edit
  user_id = current_user.id
  subject_id = Subject.where(:name => params[:name])
  @user_sub = UsersSubject.new(user_id: user_id, subject_id: subject_id)
  @user_sub.save
end

When I do this the controller populate the UsersSubject database with the correct user_id but the subject_id is always nil. The Subject database is already populate using the seed.rb file. Can someone help me understand why this is happening and help my fix it?

Thanks in advance

EDIT

Here is my development.log

Started GET "/users/edit" for ::1 at 2016-05-31 18:27:33 +0100
Processing by Users::RegistrationsController#edit as HTML
Parameters: {"utf8"=>"✓", "authenticity_token"=>"ZGvBq1uFUi1RxcInKFz1TnUs2ZzlZsP29aW1mzQBOVBTLm/Dq3C42cQQX0ksmBv95/qHnk08bG3f5u1v9taZgw==", "user"=>{"address"=>"", "city"=>"London", "postcode"=>"", "country"=>"United Kingdom", "subject"=>{"subject_id"=>"1"}}, "commit"=>"Update"}    

Moved controller code to the update action

def update
  user_id = current_user.id
  subject_id = params[:user][:subject][:subject_id] unless params[:user].nil?
  @user_sub = UsersSubject.new(user_id: user_id, subject_id: subject_id)
  @user_sub.save
end

Added the subject permission in the application_controller.rb:

class ApplicationController < ActionController::Base

  before_filter :configure_permitted_parameters, if: :devise_controller?
  protected

  def configure_permitted_parameters
    devise_parameter_sanitizer.for(:account_update) { |u| u.permit(:last_name,
      :first_name, :email, :password, :current_password, 
      subject_attributes: [:id, :subject_id]) }
  end
end

This permits the update action to access the subject_attributes

Changes in the user model

class User < ActiveRecord::Base
  has_many :users_subjects, :class_name => 'UsersSubject', dependent: :destroy
  has_many :subjects, through: :users_subjects

  accepts_nested_attributes_for :subjects
end

Answer 1

Have you looked into using the cocoon gem?

You might also want to take a look at the params you're passing into subject_id, it looks like the full object that you're grabbing from it's name. Print out the "subject_id" that you currently have.

There is a great railscasts video on this topic you might want to look into, the membership is well worth it.

Answer 2

Check your params in your rails server log. From what I guess it should be coming like this

"user" => {
  "name"=>"Michael Jackson",
  "subject" => {
    "subject_id"=>"1"
  }
}

But in your controller method, you're trying to access it like params[:name], instead you should do something like this, based on how you've nested in your form.

subject_id = params[:user][:subject][:subject_id]

subject_id is nested inside subject, which is nested inside the user object.

Hope this helps. :)

Reference code:

Do this to your edit action.

def edit
  p params #this print out the inspected params in the console log
  fail
  #this stops the code from executing further, by raising an error.
  #So we can inspect the params passed to our method in server log
  ...
end

Go to your form in your browser, and after that clear your rails server console, (Command + K) for mac, and For ubuntu, press keep pressing enter to move the current log up the screen. Now click submit, and check the server log. You'll notice something like

Started POST "/users"
Processing by UsersController#edit
Parameters: {"utf8"=>..... "user" => { "name" => "Michael Jackson", "subject" => { "subject_id" => "1" }}}

So what you want to do is, you need to create a new many to many relation between User and Subject. You're getting the user_id from devise's current_user and subject_id in your edit method from the params, as I defined. How you're receiving params might differ from how I mentioned.

Final code

def edit
  user_id = current_user.id
  subject_id = params[:user][:subject][:subject_id]
  @user_sub = UsersSubject.new(user_id: user_id, subject_id: subject_id)
  @user_sub.save
end