Add record to a has_and_belongs_to_many relationship

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

class Promotion < ActiveRecord::Base
  has_and_belongs_to_many :users

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

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 =

This results in the following error:

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

If I try this line instead: promo=

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.


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.

You can do just

User.promotions = promotion #notice that this will delete any existing promotions


User.promotions << promotion

You can read about has_and_belongs_to_many relationship here.

This is also useful = {})

so, promotion object saves, when you save User object.

And this is

User.promotion.create(attr = {})

create promotion you not need to save it or User model

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