Rails has_and_belongs_to_many migration - ruby-on-rails

Question

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.

Answer 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:

Answer 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!

Answer 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.

Answer 4

For HABTM relationships, you need to create a join table. There is only join table and that table should not have an id column. Try this migration.
def self.up
create_table :restaurants_users, :id => false do |t|
t.integer :restaurant_id
t.integer :user_id
end
end
def self.down
drop_table :restaurants_users
end
You must check this relationship rails guide tutorials