fields.py 文件源码

def __get__(self, instance, cls=None):
        if instance is None:
            return self

        # Don't use getattr(instance, self.ct_field) here because that might
        # reload the same ContentType over and over (#5570). Instead, get the
        # content type ID here, and later when the actual instance is needed,
        # use ContentType.objects.get_for_id(), which has a global cache.
        f = self.model._meta.get_field(self.ct_field)
        ct_id = getattr(instance, f.get_attname(), None)
        pk_val = getattr(instance, self.fk_field)

        try:
            rel_obj = getattr(instance, self.cache_attr)
        except AttributeError:
            rel_obj = None
        else:
            if rel_obj and (ct_id != self.get_content_type(obj=rel_obj, using=instance._state.db).id or
                            rel_obj._meta.pk.to_python(pk_val) != rel_obj._get_pk_val()):
                rel_obj = None

        if rel_obj is not None:
            return rel_obj

        if ct_id is not None:
            ct = self.get_content_type(id=ct_id, using=instance._state.db)
            try:
                rel_obj = ct.get_object_for_this_type(pk=pk_val)
            except ObjectDoesNotExist:
                pass
        setattr(instance, self.cache_attr, rel_obj)
        return rel_obj