gistfile1.py

class BookingSerializer(serializers.ModelSerializer):

    class UserOptionsField(serializers.Field):
        def field_to_native(self, obj, field_name):
            return User.objects.all().values('id', 'username')

    sales_people_options = UserOptionsField()

    class Meta:
        model = Booking
        fields = (
            'id', 'booking_no', 'current_salesperson', 'sales_people_options',
        )