add associations to exisiting models
Solution 1
belongs_to
association expect an association_id
column in its corresponding table. Since cars belongs_to user, the cars table should have a user_id
column. This can be accomplished 2 ways.
first, you can generate the column when you create the model
rails g model car user_id:references
or just add the user_id after you create the model like Richard Brown's answer. Be careful that if you use integer
instead of references
, you'd have to create the index yourself.
rails g migration add_user_id_to_cars user_id:integer
then in the generated migration, add
add_index :cars, :user_id
UPDATE:
As Joseph has mentioned in the comments, the need to add the index manually has already been addressed in the current version of Rails. I think it was introduced in Rails 4. You can read more of it in the official Rails guide for migrations. The gist of it is running the following generator
bin/rails g migration add_user_to_cars user:references
will create a migration with a line similar to
add_reference :cars, :user, index: true
This will add a user_id
column to the cars table and it will also mark that column to be indexed.
Solution 2
Following @jvnill's explanation in rails 4 (and maybe in rails 3.2 too) you can do it like this too (avoiding the id parts and remembering the exact convetions):
rails g migration AddUserToCar user:references
Which will create the following migration, taking care of both adding the column and index with all correct conventions:
class AddUserToCar < ActiveRecord::Migration
def change
add_reference :cars, :user, index: true
end
end
At the end as always run the migration:
rake db:migrate
View your schema.rb
to view the new index and user_id column.
Solution 3
Generate a migration to create the association:
rails g migration AddUserIdToCars user_id:integer
rake db:migrate
![Andrew](https://i.stack.imgur.com/vrccB.jpg?s=256&g=1)
Andrew
Software developer with strong theoretical physics background.
Updated on July 06, 2020Comments
-
Andrew almost 4 years
I'm wondering how I can add associations to my models. Suppose, I generate two models
rails generate model User rails generate model Car
Now I want to add an associations so that the models acquire the form
class User < ActiveRecord::Base has_many :cars end class Car < ActiveRecord::Base belongs_to :user end
The question is: how to apply this modification by migrations in order to obtain cars_users table in the database? I'm planning to use that table in my code.