using has_many :through and build

Question

i have three models, all for a has_many :through relationship. They look like this:

class Company < ActiveRecord::Base

  has_many :company_users, dependent: :destroy
  has_many :users, through: :company_users

  accepts_nested_attributes_for :company_users, :users

end

class CompanyUser < ActiveRecord::Base
  self.table_name = :companies_users #this is because this was originally a habtm relationship
  belongs_to :company
  belongs_to :user
end

class User < ActiveRecord::Base
  # this is a devise model, if that matters

  has_many :company_users, dependent: :destroy
  has_many :companies, through: :company_users

  accepts_nested_attributes_for :company_users, :companies

end

this loads fine, and the joins are built fine for queries. However, whenever i do something like

@company = Company.last
@user = @company.users.build(params[:user])

@user.save    #=> true
@company.save #=> true

both the User record and the CompanyUser records get created, but the company_id field in the CompanyUser record is set to NULL

INSERT INTO `companies_users` (`company_id`, `created_at`,`updated_at`, `user_id`) 
VALUES (NULL, '2012-02-19 02:09:04', '2012-02-19 02:09:04', 18)

it does the same thing when you @company.users << @user

I'm sure that I'm doing something stupid here, I just don't know what.

Answer 1

You can't use a has_many :through like that, you have to do it like this:

@company = Company.last
@user    = User.create( params[:user] ) 
@company.company_users.create( :user_id => @user.id )

Then you will have the association defined correctly.

update

In the case of the comment below, as you already have accepts_nested_attributes_for, your parameters would have to come like this:

{ :company => 
    { :company_users_attributes => 
        [ 
          { :company_id => 1, :user_id => 1 } ,
          { :company_id => 1, :user_id => 2 },
          { :company_id => 1, :user_id => 3 } 
        ]
    } 
}

And you would have users being added to companies automatically for you.