Tastypie - make fields read only for object update but not for creation

base.py

from tastypie.resources import ModelResource as BaseModelResource


class ReadOnlyFieldsAfterAddMeta(BaseModelResource.__metaclass__):

    def __new__(cls, name, bases, attrs):
        new_class = super(ReadOnlyFieldsAfterAddMeta, cls).__new__(cls, name, bases, attrs)
        read_only_fields_after_add = getattr(new_class._meta, 'read_only_after_add', ())
        for field_name in read_only_fields_after_add:

            # Because scope
            def make_hydrator(field_name):
                def hydrate_read_only_after_add(self, bundle):
                    if bundle.obj.pk:
                        # Rubbish test for newness - but it's the standard way Tastypie does it
                        if field_name in bundle.data:
                            del bundle.data[field_name]
                    return bundle

            hydrate_read_only_after_add = make_hydrator(field_name)
            method_name = 'hydrate_{0}'.format(field_name)
            setattr(new_class, method_name, hydrate_read_only_after_add)

        return new_class


class ModelResource(BaseModelResource):

    __metaclass__ = ReadOnlyFieldsAfterAddMeta


class MyResource(ModelResource):

    class Meta:
        queryset = MyModel.objects.all()
        read_only_after_add = ('spam', 'ham',)