Add record to a has_and_belongs_to_many relationship

Question

I have two models, users and promotions. The idea is that a promotion can have many users, and a user can have many promotions.

class User < ActiveRecord::Base   has_and_belongs_to_many :promotions end  class Promotion < ActiveRecord::Base   has_and_belongs_to_many :users end 

I also have a promotions_users table/model, with no id of its own. It references user_id and promotions_id

class PromotionsUsers < ActiveRecord::Base end 

So, how do I add a user to a promotion? I've tried something like this:

user = User.find(params[:id]) promotion = Promotion.find(params[:promo_id]) promo = user.promotions.new(promo) 

This results in the following error:

NoMethodError: undefined method `stringify_keys!' for #<Promotion:0x10514d420> 

If I try this line instead: promo= user.promotions.new(promo.id)

I get this error:

TypeError: can't dup Fixnum 

I'm sure that there is a very easy solution to my problem, and I'm just not searching for the solution the right way.

Answer 1

user = User.find(params[:id]) promotion = Promotion.find(params[:promo_id]) user.promotions << promotion 

user.promotions is an array of the promotions tied to the user.

See the apidock for all the different functions you have available.