How to generate associations between models

Question

I am wondering how to do the association in Rails correct. First I create a City model and an Organisation. Now I want to have an Organisation have a City... this is done by adding the has_many and has_one associations. After that I run rake db:migrate. But somehow it does not create a field city or city_id in my database model. Do I have to do this myself? Shouldn't rails now create a foreign key constraint in the database?

To see if it has worked I am using rails c and type in Organisation the answer is the following:

=> Organisation(id: integer, name: string, description: string, url: string, created_at: datetime, updated_at: datetime) 

Please excuse my stupid question... I am a beginner in Rails and everything is still very unfamiliar.

Thanks!

---

City:

class City < ActiveRecord::Base
  has_many :organisations
end

Organisation:

class Organisation < ActiveRecord::Base
  has_one :city
end

Create City:

class CreateCities < ActiveRecord::Migration
  def change
    create_table :cities do |t|
      t.string :name
      t.string :country

      t.timestamps
    end
  end
end

Create Organisation:

class CreateOrganisations < ActiveRecord::Migration
  def change
    create_table :organisations do |t|
      t.string :name
      t.string :description
      t.string :url

      t.timestamps
    end
  end
end

Answer 1

There are a couple things wrong with this.

1. You need to specify a belongs_to on the other side of a has_many or has_one association. The model that defines a belongs_to association is where the foreign key belongs.

   So if an Organization has_one :city, then a City needs to belongs_to :organization. Alternatively, if a City has_one :organization, then the Organization needs to belongs_to :city.

   Looking at your setup, it looks like you want the belongs_to definition inside the City model.

2. The migrations aren't built off the model definitions. Instead, they are built from the db/migrations folder. A migration is created when you run the rails g model command (or rails g migration). In order to get a foreign key, you need to tell the generator to create it.

   rails generate model organization name:string description:string url:string city_id:integer
   

   Or

   rails generate model city name:string description:string url:string organization_id:integer