Trek | Has Many, with Precedence

Every so often I have a has_many ActiveRecord relationship with a twist: one (and only one) of the many possible related items is special in some way and I want to be get and set it like other associations. Imagine a Customer has several phone numbers, but only one is their primary contact number.

ActiveRecord has a few strategies for multiple, similar relationships. You could use association extensions and define a sub-assocation like so:

class Customer < ActiveRecord::Base
  has_many :phone_numbers do
    def priamry
      find(:first, :conditions => {:is_primary => true})
    end
  end
end

which will allow you to retrieve the primary phone number, but not set it:

c = Customer.find(1)
c.phone_numbers.primary
# => <PhoneNumber id: 1 number: '5551234', is_primary true>
c.phone_numbers.primary = PhoneNumber.new(:number => '5551235')
# => NoMethodError: undefined method `primary='

ActiveRecord lets you define multiple relationships to the same class with some configuration:

class Customer < ActiveRecord::Base
  has_many :phone_numbers
  has_many :primary_phone_numbers, 
           :class_name => 'PhoneNumber', 
           :conditions => {:is_primary => true}
end

This will let you ask for either collection, assign new or existing objects to either collection, and even use the build and create methods on the collection to get new objects with the attributes pre-assigned to match the :conditions you supplied:

c = Customer.find(1)
c.phone_numbers
#    [<PhoneNumber id: 1 number: '5551234', is_primary true,
#    <PhoneNumber id: 2 number: '5551234', is_primary false >]
c.primary_phone_numbers
#    [<PhoneNumber id: 1 number: '5551234', is_primary true]

The problem here is that you always get a collection back, and will need additional code to enforce the existence of a single primary phone number for a customer.

c = Customer.find(1)
c.phone_numbers
#    [<PhoneNumber id: 1 number: '5551234', is_primary true,
#    <PhoneNumber id: 2 number: '5551234', is_primary false >]
c.primary_phone_numbers << PhoneNumber.new(:number => '555 9081')
#    [<PhoneNumber id: 3 number: '5559081', is_primary true]
c.primary_phone_numbers
# two primary phone nubmers!
# [<PhoneNumber id: 1 number: '5551234', is_primary true,
#   <PhoneNumber id: 3 number: '5559081', is_primary true]

You might think, at first, the solution is to has has_one

class Customer < ActiveRecord::Base
  has_many :phone_numbers
  has_one  :primary_phone_number, 
           :class_name => 'PhoneNumber', 
           :conditions => {:is_primary => true}
end

This will work fine for retrieving, but you'll run into trouble when trying to assign. Part of the job of a has_one relationship is ensuring that the foreign key appears only once for the type of association.

c = Customer.find(1)
c.phone_numbers
#    [<PhoneNumber id: 1 number: '5551234', is_primary true,
#    <PhoneNumber id: 2 number: '5551234', is_primary false >]
c.primary_phone_number 
#    <PhoneNumber id: 1 number: '5551234', is_primary true>
c.primary_phone_number = Number.new(:number => '555 0919')
#    <PhoneNumber id: 3 number: '5550919', is_primary true>
c.reload
c.phone_numbers
# foreign keys cleared
# [<PhoneNumber id: 3 number: '5550919', is_primary true>]

The solution I'm currently using is to flip the relationship (and where the foreign key is stored) by making a Customer belong_to primary PhoneNumber

class Customer < ActiveRecord::Base
  has_many :phone_numbers
  belongs_to  :primary_phone_number, :class_name => 'PhoneNumber'
end

Now we can get and set a primary phone number without overwriting the old ones. Unfortunately we have the complementary problem to the one above. When we ask for phone_numbers the collection won't include the primary phone number (since it lacks the customer_id in the phone_numbers table)

  c.phone_numbers
  #    [<PhoneNumber id: 1 number: '5551234'>]
  c.primary_phone_number 
  #    nil
  c.primary_phone_number = Number.new(:number => '555 0919')
  #    <PhoneNumber id: 3 number: '5550919'>
  c.reload
  c.phone_numbers
  # doesn't include the primary phone number
  # [<PhoneNumber id: 1 number: '5551234'>]

In the future, We'll be able to solve this with a before_add callback whenever this patch is applied (right now they only work for has_many associations):

belongs_to  :primary_phone_number, 
            :class_name => 'PhoneNumber', 
            :before_add => :add_to_phone_numbers

def add_to_phone_numbers(phone_number)
  self.phone_numbers << phone_number
end

Until then, you can work around the limitation with some alias_method_chain trickery:

belongs_to :phone_number

def phone_number_with_callback=(phone_number)
  self.phone_numbers << phone_number
  self.phone_number_without_callback = phone_number
end

alias_method_chain :phone_number=, :callback

Some writing by Trek

& archive, twitter, github

Has Many, with Precedence