Creating a many to many relationship in Rails

Question

This is a simplified example of what I am trying to achieve, I'm relatively new to Rails and am struggling to get my head around relationships between models.

I have two models, the User model and the Category model. A user can be associated with many categories. A particular category can appear in the category list for many users. If a particular category is deleted, this should be reflected in the category list for a user.

In this example:

My Categories table contains five categories:

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
| ID | Name                       |
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
| 1  | Sports                     | 
| 2  | News                       |
| 3  | Entertainment              |
| 4  | Technology                 |
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

My Users table contains two users:

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
| ID | Name                       |
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
| 1  | UserA                      | 
| 2  | UserB                      |
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

UserA may choose Sports and Technology as his categories

UserB may choose News, Sports and Entertainment

The sports category is deleted, both UserA and UserB category lists reflect the deletion

I've toyed around with creating a UserCategories table which holds the ids of both a category and user. This kind of worked, I could look up the category names but I couldn't get a cascading delete to work and the whole solution just seemed wrong.

The examples of using the belongs_to and has_many functions that I have found seem to discuss mapping a one-to-one relationship. For example, comments on a blog post.

• How do you represent this many-to-many relationship using the built-in Rails functionality?
• Is using a separate table between the two a viable solution when using Rails?

Answer 1

You want a has_and_belongs_to_many relationship. The guide does a great job of describing how this works with charts and everything:

http://guides.rubyonrails.org/association_basics.html#the-has-and-belongs-to-many-association

You will end up with something like this:

# app/models/category.rb
class Category < ActiveRecord::Base
  has_and_belongs_to_many :users
end

# app/models/user.rb
class User < ActiveRecord::Base
  has_and_belongs_to_many :categories
end

Now you need to create a join table for Rails to use. Rails will not do this automatically for you. This is effectively a table with a reference to each of Categories and Users, and no primary key.

Generate a migration from the CLI like this:

bin/rails g migration CreateCategoriesUsersJoinTable

Then open it up and edit it to match:

For Rails 4.0.2+ (including Rails 5.2):

def change
  # This is enough; you don't need to worry about order
  create_join_table :categories, :users

  # If you want to add an index for faster querying through this join:
  create_join_table :categories, :users do |t|
    t.index :category_id
    t.index :user_id
  end
end

Rails < 4.0.2:

def self.up
  # Model names in alphabetical order (e.g. a_b)
  create_table :categories_users, :id => false do |t|
    t.integer :category_id
    t.integer :user_id
  end

  add_index :categories_users, [:category_id, :user_id]
end

def self.down
  drop_table :categories_users
end

With that in place, run your migrations and you can connect Categories and Users with all of the convenient accessors you're used to:

User.categories  #=> [<Category @name="Sports">, ...]
Category.users   #=> [<User @name="UserA">, ...]
User.categories.empty?