old_contents: # Python 2 and 3
  from __future__ import unicode_literals
  
  from gapipy.resources.checkin import Checkin
  
  from ..base import Resource
  from .transaction import Payment, Refund
  from .document import Invoice, Document
  from .override import Override
  from .service import Service
  
  
  class Booking(Resource):
      _resource_name = 'bookings'
      _is_parent_resource = True
  
      _as_is_fields = ['id', 'href', 'external_id', 'currency']
      _price_fields = [
          'amount_owing',
          'amount_paid',
          'amount_pending',
          'commission',
          'tax_on_commission',
      ]
      _date_fields = [
          'date_closed', 'date_of_first_travel', 'date_of_last_travel',
          'balance_due_date',
      ]
      _date_time_fields_utc = ['date_created', ]
      _resource_fields = [
          ('agent', 'Agent'),
          ('agency', 'Agency'),
          ('associated_agency', 'Agency'),
      ]
  
      @property
      def _resource_collection_fields(self):
          return [
              ('services', Service),
              ('invoices', Invoice),
              ('payments', Payment),
              ('refunds', Refund),
              ('documents', Document),
              ('overrides', Override),
              ('checkins', Checkin),
          ]
  
new_contents: # Python 2 and 3
  from __future__ import unicode_literals
  
  from gapipy.resources.checkin import Checkin
  
  from ..base import Resource
  from .agency_chain import AgencyChain
  from .document import Invoice, Document
  from .override import Override
  from .service import Service
  from .transaction import Payment, Refund
  
  
  class Booking(Resource):
      _resource_name = 'bookings'
      _is_parent_resource = True
  
      _as_is_fields = ['id', 'href', 'external_id', 'currency']
      _price_fields = [
          'amount_owing',
          'amount_paid',
          'amount_pending',
          'commission',
          'tax_on_commission',
      ]
      _date_fields = [
          'date_closed', 'date_of_first_travel', 'date_of_last_travel',
          'balance_due_date',
      ]
      _date_time_fields_utc = ['date_created', ]
      _resource_fields = [
          ('agency', 'Agency'),
          ('agency_chain', AgencyChain),
          ('agent', 'Agent'),
          ('associated_agency', 'Agency'),
      ]
  
      @property
      def _resource_collection_fields(self):
          return [
              ('services', Service),
              ('invoices', Invoice),
              ('payments', Payment),
              ('refunds', Refund),
              ('documents', Document),
              ('overrides', Override),
              ('checkins', Checkin),
          ]
  
subject: Add agency chain to Booking