Creating a many to many relationship in Rails

Just complementing coreyward's answer above: If you already have a model that has a belongs_to, has_many relation and you want to create a new relation has_and_belongs_to_many using the same table you will need to:

rails g migration CreateJoinTableUsersCategories users categories

Then,

rake db:migrate

After that, you will need to define your relations:

User.rb:

class Region < ApplicationRecord
  has_and_belongs_to_many :categories
end

Category.rb

class Facility < ApplicationRecord
  has_and_belongs_to_many :users
end

In order to populate the new join table with the old data, you will need to in your console:

User.all.find_each do |u|
  Category.where(user_id: u.id).find_each do |c|
    u.categories <<  c
  end
end

You can either leave the user_id column and category_id column from the Category and User tables or create a migration to delete it.

The most popular is 'Mono-transitive Association', you can do this:

class Book < ApplicationRecord
  has_many :book_authors
  has_many :authors, through: :book_authors
end

# in between
class BookAuthor < ApplicationRecord
  belongs_to :book
  belongs_to :author
end

class Author < ApplicationRecord
  has_many :book_authors
  has_many :books, through: :book_authors
end

A has_many :through association is often used to set up a many-to-many connection with another model. This association indicates that the declaring model can be matched with zero or more instances of another model by proceeding through a third model. For example, consider a medical practice where patients make appointments to see physicians. Ref.: https://guides.rubyonrails.org/association_basics.html#the-has-many-through-association

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?