Nested forms with has_one :through

Question

I'm having some trouble to implement a nested form with a has_one :through association.

Models

# model: member.rb

belongs_to :user
has_one :academic

# model: user.rb

has_one :member
has_one :academic, through: :member

accepts_nested_attributes_for :member, reject_if: :all_blank 
accepts_nested_attributes_for :academic, reject_if: :all_blank

# model: academic.rb

belongs_to :member
belongs_to :user 

Controller

# users_controller.rb

def new
  @user = User.new
  @user.build_member
  @user.build_academic  
end

I also have tried with:

@user.member.build_academic

View

# new.html.erb

<%= simple_form_for(resource, as: resource_name, url: registration_path(resource_name)) do |ff| %>

  <%= ff.text_field :email %>

# member belongs to user so I can call a fields_for
<% ff.fields_for :member do |f| %>

  <%= f.text_field :name %>

# this part is not shown. What is wrong with my association?
<% f.fields_for :academic do |a| %>

  <%= a.text_field :major %>

<% end %>
 <% end %>
  <% end %>

I've taken a look into the Rails documentation. The first fields_for is shown in the page (:member), but the second one (:academic), which has the has_one :through association, is not shown in the page.

Any help will be appreciated. Thank you.

Answer 1

For future reference or for anyone that has the same problem, I found a way around to fix this problem, in case you have same structure.

I could fix this by going to the view, before the form, and writing:

<% resource.member.build_academic %>

In my case resource is User, set by devise, a Rails gem used for authentication.

And you don't need to reference any :through or whatsoever in your model.

It is not the most efficient way, but I haven't found any other solution. Hope it helps.

Answer 2

Through

If you want to build your data through a relation, you have to pass the associated data as it's constructed:

#app/models/member.rb
class Member < ActiveRecord::Base
   belongs_to :user
   has_one :academic
   
   accepts_nested_attributes_for :academic
end

#app/models/user.rb
class User < ActiveRecord::Base
   has_one :member
   has_one :academic, through: :member

   accepts_nested_attributes_for :member
end

#app/models/academic.rb
class Academic < ActiveRecord::Base
   belongs_to :member
   belongs_to :user
end

This will allow you to do the following:

#app/controllers/members_controller.rb
class MembersController < ApplicationController
   def new
      @member = Member.new
      @member.build_member.build_academic
   end

   def create
      @member = Member.new member_params
      @member.save
   end

   private

   def member_params
      params.require(:member).permit(:x, :y, :z, academic_attributes: [:some, :attributes, member_attributes:[...]])
   end
end

This would permit the following:

#app/views/users/new.html.erb
<%= form_for @user do |f| %>
   <%= f.fields_for :member do |m| %>
      <%= f.text_field :name %>
      ...
      <%= m.fields_for :academic do |a| %>
           <% a.text_field :name %>
           ...
      <% end %> 
   <% end %>
   <%= f.submit %>
<% end %>

This works to build a new member object, and a new academic object from the user. Although not strictly what you're asking, it looks like it could benefit you in some form.

---

Associations

If you want to do what you're asking (IE build_member and build_academic exclusively), you'll need to get rid of the has_one :through relationship...

#app/models/user.rb
class User < ActiveRecord::Base
   has_many :memberships
   has_many :academics, through: :memberships
end

#app/models/membership.rb
class Membership < ActiveRecord::Base
   belongs_to :user
   belongs_to :academic
end

#app/models/academic.rb
class Academic < ActiveRecord::Base
   has_many :memberships
   has_many :users, through: :memberships
end

The problem is you're basically trying to build a relationship for a direct association (member) and an indirect relationship (academic).

If you want to build both exclusively, you have to make them have a direct association with your main model. The above should allow you to do the following:

#app/controllers/users_controller.rb
class UsersController < ApplicationController
   def new 
       @user = User.new
       @user.members.build.academics.build
   end
end

Much like my top example, this will pass nested data through your form - if you wanted to have it completely exclusively, do this:

#app/models/user.rb
class User < ActiveRecord::Base
   has_one :member
   has_one :academic
   has_and_belongs_to_many :academics
end

#app/models/member.rb
class Member < ActiveRecord::Base
   belongs_to :user
end

#app/models/academic.rb
class Academic < ActiveRecord::Base
   belongs_to :user
   has_and_belongs_to_many :users
end

This will allow the following:

#app/controllers/users_controller.rb
class UsersController < ApplicationController
   def new
      @user = User.new
      @user.build_member
      @user.build_academic
   end

   def create
      @user = User.new user_params
      @user.save
   end

   private

   def user_params
      params.require(:user).permit(:user, :params, member_attributes: [], academic_attributes:[])
   end
end