Question

I working in Rails 3 Activerecord. I have two models City and Shipment and I’m trying to model a shipment leaving from one city (ship_from) and shipping to another (ship_to). This is what I have:

class Shipment < ActiveRecord::Base
  belongs_to :ship_from, :class_name => "City", :foreign_key => "city_id"
  belongs_to :ship_to, :class_name => "City", :foreign_key => "city_id"
end

class City < ActiveRecord::Base
  has_many :ship_froms
  has_many :ship_tos
end

I know I missing something very simple and obvious, but I’m just not getting it. Any guidance would be greatly appreciated. Thanks.

Answer 1

You’re modeling two one-to-many relations. You’re saying as Shipment has one from city and one to city.

The one side is the part that’s saved in your table. So you’re saving a from_city_id and a to_city_id in your shipments table. Both these columns contain the id of a City.

You model this one city relation by adding the following code in the Shipment class:

belongs_to :from_city, :class_name => 'City'

This code means we have a property from_city which type is City (from option :class_name, default would be FromCity). You can find the City by searching the table cities for id = from_city_id (id is the default for the option :primary_key, :from_city_id is the default for :foreign_key).

For the to_city you declare

belongs_to :to_city, :class_name => ‘City’

The many part of the relation is in the City class. To say you have many Shipments from this city. You declare the following:

many :from_shipments, :class_name => 'Shipment', :foreign_key => 'from_city_id'

The code means we have a property from_shipments which type is a collection of Shipments (:class_name). You can find the Shipment by searching in the table shipments for from_city_id = id (we override the default city_id by declaring a :foreign_key)

For the to_city you declare

many :to_shipments, :class_name => 'Shipment', :foreign_key => 'to_city_id'

More info about the defaults and options for belongs_to, has_one, has_many can be found here.