old_contents: # Python 2 and 3
  from __future__ import unicode_literals
  
  from gapipy.models import Address
  from gapipy.models.base import BaseModel
  from gapipy.resources.base import Resource
  
  
  class MedicalDetail(BaseModel):
      _as_is_fields = ['type', 'value']
  
  
  class MembershipProgram(BaseModel):
      _as_is_fields = ['code', 'label', 'value']
  
  
  class Customer(Resource):
      _resource_name = 'customers'
      _is_listable = False
      _is_parent_resource = True
  
      _as_is_fields = [
          'account_email',
          'emergency_contacts',
          'gender',
          'href',
          'id',
          'meal_notes',
          'meal_preference',
          'medical_notes',
          'name',
          'nationality',
          'passport',
          'phone_numbers',
          'place_of_birth',
      ]
  
      _date_fields = ['date_of_birth', ]
  
      _model_fields = [
          ('address', Address),
      ]
  
      _model_collection_fields = [
          ('medical_details', MedicalDetail),
          ('membership_programs', MembershipProgram),
      ]
  
      @property
      def _resource_collection_fields(self):
          from .booking import Booking
          return [
              ('bookings', Booking),
          ]
  
new_contents: # Python 2 and 3
  from __future__ import unicode_literals
  
  from gapipy.models import Address
  from gapipy.models.base import BaseModel
  from gapipy.resources.base import Resource
  
  from ..geo import Nationality
  
  
  class MedicalDetail(BaseModel):
      _as_is_fields = ['type', 'value']
  
  
  class MembershipProgram(BaseModel):
      _as_is_fields = ['code', 'label', 'value']
  
  
  class Customer(Resource):
      _resource_name = 'customers'
      _is_listable = False
      _is_parent_resource = True
  
      _as_is_fields = [
          'account_email',
          'emergency_contacts',
          'gender',
          'href',
          'id',
          'meal_notes',
          'meal_preference',
          'medical_notes',
          'name',
          'passport',
          'phone_numbers',
          'place_of_birth',
      ]
  
      _date_fields = ['date_of_birth', ]
  
      _resource_fields = [
          ('nationality', Nationality),
      ]
  
      _model_fields = [
          ('address', Address),
      ]
  
      _model_collection_fields = [
          ('medical_details', MedicalDetail),
          ('membership_programs', MembershipProgram),
      ]
  
      @property
      def _resource_collection_fields(self):
          from .booking import Booking
          return [
              ('bookings', Booking),
          ]
  
subject: make Customer.nationality a "resource field"