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, depende         


        

Answer 1

If you have a has_many :through association and you want to save an association using build you can accomplish this using the :inverse_of option on the belongs_to association in the Join Model

Here's a modified example from the rails docs where tags has a has_many :through association with posts and the developer is attempting to save tags through the join model (PostTag) using the build method:

@post = Post.first
@tag = @post.tags.build name: "ruby"
@tag.save

The common expectation is that the last line should save the "through" record in the join table (post_tags). However, this will not work by default. This will only work if the :inverse_of is set:

class PostTag < ActiveRecord::Base
  belongs_to :post
  belongs_to :tag, inverse_of: :post_tags # add inverse_of option
end

class Post < ActiveRecord::Base
  has_many :post_tags
  has_many :tags, through: :post_tags 
end

class Tag < ActiveRecord::Base
  has_many :post_tags
  has_many :posts, through: :post_tags  
end

So for the question above, setting the :inverse_of option on the belongs_to :user association in the Join Model (CompanyUser) like this:

class CompanyUser < ActiveRecord::Base
  belongs_to :company
  belongs_to :user, inverse_of: :company_users
end

will result in the following code correctly creating a record in the join table (company_users)

company = Company.first
company.users.build(name: "James")
company.save

Source: here & here