One to Many BD - foriegn key & updates - rails

Question

In my rails app i have a two models model customer (id, name, email) and cars(id, model, year).

customer has many cars cars belong to customer

1) I want to add the customer id field to the cars model. Will adding t.integer :customer_id to the migration file be enough?

2) Once added how would i start populating the said cars db?

(I just started on rails last week so would appreciate any help)

Answer 1

It is possible to add to your existing migration, but a more common workflow is to create a second migration to change an existing table.

Read about Rails Migrations for all the details.

1. Make a migration

   rails generate migration add-customer-id-to-cars
   

2. Add the correct code

   class AddCustomerIdToCars < ActiveRecord::Migration
     def change
       add_column :cars, :customer_id, :integer
     end
   end
   

3. Migrate

   rake db:migrate
   

4. Add the association in car.rb

   class Car < ActiveRecord::Base
     # Your Car code
   
     belongs_to :customer
   end
   

5. Add the inverse association in your customer.rb

   class Customer < ActiveRecord::Base
     # Your Customer code
   
     has_many :cars
   end
   

Answer 2

(1) Yes. Though it would be more idiomatic to use t.references.

From Rails guides (adapted to context):

Using t.integer :customer_id makes the foreign key naming obvious and explicit. In current versions of Rails, you can abstract away this implementation detail by using t.references :customer instead.

You will also need to specify the associations in the model file.

class Customer < ActiveRecord::Base
  has_many :cars
end

class Car < ActiveRecord::Base
  belongs_to :customer
end

(2) You can use something like:

customer = Customer.find(id)
car = customer.cars.new
car.model = "some model"
car.save!