Tweetegy On the edge of chaos with Ruby, Rails, CoffeeScript and Angular JS.

Setting join table attribute :has_many :through association in Rails ActiveRecord

I needed to set an attribute on a join table or in ActiveRecord, the* :through* model as an atomic save action. Let’s set the scene: I have a Product *and a *User *which has a many-to-many relationship since a *Product *can have many *Users *and a User can have many *Products.

Typically, in a database we might have a table product, user and product_user. In Rails I created an association class called Collaborators *since it fit my domain better. Additionally the *Collaborators *class has one attribute *is_admin which indicates the User *(or *Collaborator *actually) is an administrator for the particular *Product.

One of the business rules in the application was that if a User *creates a new *Product *then they are automatically an administrator for that *Product. My dilemma was how to create the new Product, associate it correctly with the User *\and* set* is_admin* to true all in one nice and neat command?

The answer, it turns out, is straightforward and elegant. I am fairly new to Rails myself, so I understand if this might appear a little overwhelming if you are also just starting out. I have included a set of useful references at the end of this post to help you on your way.

1 class Product< ActiveRecord::Base
2   has_many :collaborators
3   has_many :users, :through => :collaborators
4 end
1 class User< ActiveRecord::Base
2   has_many :collaborators
3   has_many :products, :through => :collaborators
4 end
1 class Collaborator < ActiveRecord::Base
2   belongs_to :product
3   belongs_to :user
4 end

Once you have the above ActiveRecord models setup all you need to do is execute the following code to save a new Product, associate a User *as a *Collaborator *and set them as an administrator for the *Product.

1 #assume we have the product and current_user instance already set
2 product.save && product.collaborators.create(:user => current_user, :is_admin => true)

Useful Resources and Docs for ActiveRecord Associations Has Many Through Association ActiveRecord::Base Documentation