Rails 4: polymorphism, single-table inheritance, scoping...? - ruby-on-rails-4

Question

Suppose the following:
class Customer < ActiveRecord::Base
has_many :orders
end
class Order < ActiveRecord::Base
belongs_to :customer
end
Customers can be red customers and/or blue customers, but they are never just a customer. Red customers own dogs, blue customers wear hats (a customer must be hat-wearing or dog-owning). For a given order, red_customer is never equal to blue_customer. Since I never use customer_id, I want to eliminate it from my schema and replace it with red_customer_id and blue_customer_id. Each order has a red_customer_id and blue_customer_id.
Examples...
#some_order.customer # no, bad, do not want
#some_order.red_customer # returns a user object
#some_order.blue_customer # returns a user object
Schema (currently ugly)
create_table "orders"
t.string "name"
t.integer "user_id" # I want to get rid of this
t.integer "red_customer_id"
t.integer "blue_customer_id"
end
But if I remove user_id, then my associations break. How can I clean this up?

Answer 1

Assuming an order can only belong to a single user at a time I do not recommended to have 2 foreign_keys in your table. You can achieve the same result using Single Table Inheritance:
class Customer < ActiveRecord::Base
has_many :orders
end
class RedCustomer < Customer
end
class BlueCustomer < Customer
end
class Order < ActiveRecord::Base
belongs_to :red_customer, foreign_key: 'customer_id'
belongs_to :blue_customer, foreign_key: 'customer_id'
end
This way you keep a single table for customers and a single foreign_key on orders.
For this to work you need a type string column in the customer table, also as the Order migration, add polymorphic: true to the customer foreign_key declaration, this way the column customer_type will also be created.
class CreateCustomers < ActiveRecord::Migration
def change
create_table :customers do |t|
t.string :type
t.timestamps null: false
end
end
end
class CreateOrders < ActiveRecord::Migration
def change
create_table :orders do |t|
t.references :customer, polymorphic: true, index: true
t.timestamps null: false
end
end
end
It works as expected:
order = Order.create
order.red_customer = BlueCustomer.create #=> raises <ActiveRecord::AssociationTypeMismatch>
order.red_customer = RedCustomer.create #=> Ok
order.blue_customer #=> nil
order.red_customer #=> #<RedCustomer id: ...>
order.blue_customer = BlueCustomer.create #=> Ok
order.red_customer #=> nil
Hope this helps you.