Rails Model, belongs to many

associations, model-associations, ruby-on-rails, ruby-on-rails-3

Solution

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!

Problem

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.

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