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.

score 53 · Accepted Answer · edited Jul 03 '20 at 07:40

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!

that was really helpful! One useful addition would be some examples on how we can access these relationships in the code (either in a controller or in a view) thank you — msanjay, Feb 14 '14 at 08:51

Rohit · Answer 2 · 2014-02-18T06:18:34.723

35

This is what @Jaime Bellmyer used

# 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

I would recommend using this

# app/models/category.rb
class Category < ActiveRecord::Base
  has_many :category_items
  has_many :items, :through => :category_items
end

# app/models/item.rb
class Item < ActiveRecord::Base
  has_many :category_items
  has_many :categories, :through => :category_items
end

# app/models/category_items.rb
class CategoryItems < ActiveRecord::Base
  belongs_to :category
  belongs_to :items
end

If you use this you will have a join model which will give you more control over handling Category and Item. But using what @Jaime suggested you will have only a join table and not a model, which will not be under control.

edited Feb 18 '14 at 06:18

answered Dec 09 '10 at 07:46

Rohit

5,631
4
31
59

does this mean I should take out the foreign key out of the tables since they are linked by the join table? – bigpotato Oct 01 '12 at 15:52
Yes. You won't need to store the foreign keys in the parent tables, those can be stored in the join model. – Rohit Oct 03 '12 at 06:12
Why does it not suffice to simply say Category has_many :items (an item belongs_to_many categories but ActiveRecord doesn't have this) – Devin Rhode Oct 19 '14 at 04:45
Also, say you wanted to layer in users, and they can have a personal library of items. Would this be a User has_many :category_items and has_many :items, :through => :category_items? – Devin Rhode Oct 19 '14 at 04:46
Lastly, would it be smart or foolish to just say a User has_one :category, representing their library as it's own category? Would you want to make category polymorphic maybe? – Devin Rhode Oct 19 '14 at 04:46

Rails Model, belongs to many

2 Answers2

Linked