Rails Model, belongs to many

Question

I'm having a hard time figuring out how to association one of my models with multiple of another.

As it is now, I have:

class ModelA < ActiveRecord::Base   has_many :model_b end  class ModelB < ActiveRecord::Base   belongs_to :model_a end 

However... ModelB needs to belong to not only one instance of ModelA, but possibly three. I know there is a has_many :through, but I'm not sure how it would work in this case. EVERY instance of ModelA will always have exactly three instances of ModelB. But as said before, ModelB can belong to more than just one instance of ModelA.

Answer 1

Many-to-many relationships in rails don't use belongs_to. Instead, you want to use one of a couple options. The first is has_and_belongs_to_many:

# app/models/category.rb class Category < ActiveRecord::Base   has_and_belongs_to_many :items end  # app/models/item.rb class Item < ActiveRecord::Base   has_and_belongs_to_many :categories end 

And you'll need to add an extra join table in your database, with a migration like this:

class AddCategoriesItems < ActiveRecord::Migration   def self.up     create_table :categories_items, :id => false do |t|       t.integer :category_id       t.integer :item_id     end   end    def self.down     drop_table :categories_items   end end 

You can see that the join table's name is the combination of the two other tables' names. The tables must be mentioned in alphabetical order as above, and the :id => false needs to be there, since we don't want a primary key on this table. It will break the rails association.

There's also another, more complex method known as has_many :through if you need to store info about the relationship itself. I've written a whole article detailing how to do both methods, and when to use each:

Basic many-to-many Associations in Rails

I hope this helps, and contact me if you have any other questions!