add associations to exisiting models
associations, migration, ruby-on-rails
Solution
`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.
Problem
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.