using has_many :through and build - ruby-on-rails

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.

Answer 2

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

Answer 3

I suspect your params[:user] parameter, otherwise your code seems clean. We can use build method with 1..n and n..n associations too, see here.
I suggest you to first make sure that your model associations works fine, for that open the console and try the following,
> company = Company.last
=> #<Tcompany id: 1....>
> company.users
=> []
> company.users.build(:name => "Jake")
=> > #<User id: nil, name: "Jake">
> company.save
=> true
Now if the records are being saved fine, debug the parameters you pass to build method.
Happy debugging :)