Rails has_and_belongs_to_many migration

I have two models restaurant and user that I want to perform a has_and_belongs_to_many relationship.

I have already gone into the model files and added the has_and_belongs_to_many :restaurants and has_and_belongs_to_many :users

I assume at this point I should be able to do something like with Rails 3:

rails generate migration ....

but everything I have tried seems to fail. I'm sure this is something really simple I'm new to rails so I'm still learning.

Solution 1:

You need to add a separate join table with only a restaurant_id and user_id (no primary key), in alphabetical order.

First run your migrations, then edit the generated migration file.

Rails 3

rails g migration create_restaurants_users_table

Rails 4:

rails g migration create_restaurants_users

Rails 5

rails g migration CreateJoinTableRestaurantUser restaurants users

From the docs:

There is also a generator which will produce join tables if JoinTable is part of the name:

Your migration file (note the :id => false; it's what prevents the creation of a primary key):

Rails 3

class CreateRestaurantsUsers < ActiveRecord::Migration
  def self.up
    create_table :restaurants_users, :id => false do |t|
        t.references :restaurant
        t.references :user
    end
    add_index :restaurants_users, [:restaurant_id, :user_id]
    add_index :restaurants_users, :user_id
  end

  def self.down
    drop_table :restaurants_users
  end
end

Rails 4

class CreateRestaurantsUsers < ActiveRecord::Migration
  def change
    create_table :restaurants_users, id: false do |t|
      t.belongs_to :restaurant
      t.belongs_to :user
    end
  end
end

t.belongs_to will automatically create the necessary indices. def change will auto detect a forward or rollback migration, no need for up/down.

Rails 5

create_join_table :restaurants, :users do |t|
  t.index [:restaurant_id, :user_id]
end

Note: There is also an option for a custom table name that can be passed as a parameter to create_join_table called table_name. From the docs

By default, the name of the join table comes from the union of the first two arguments provided to create_join_table, in alphabetical order. To customize the name of the table, provide a :table_name option:

Solution 2:

The answers here are quite dated. As of Rails 4.0.2, your migrations make use of create_join_table.

To create the migration, run:

rails g migration CreateJoinTableRestaurantsUsers restaurant user

This will generate the following:

class CreateJoinTableRestaurantsUsers < ActiveRecord::Migration
  def change
    create_join_table :restaurants, :users do |t|
      # t.index [:restaurant_id, :user_id]
      # t.index [:user_id, :restaurant_id]
    end
  end
end

If you want to index these columns, uncomment the respective lines and you're good to go!

Solution 3:

When creating the join table, pay careful attention to the requirement that the two tables need to be listed in alphabetical order in the migration name/class. This can easily bite you if your model names are similar, e.g. "abc" and "abb". If you were to run

rails g migration create_abc_abb_table

Your relations will not work as expected. You must use

rails g migration create_abb_abc_table

instead.