using has_many :through and build

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.

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 optionendclass Post < ActiveRecord::Base  has_many :post_tags  has_many :tags, through: :post_tags endclass 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_usersend

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

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

Source: here & here

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 :)