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"""
Logical deletion for Django models. Uses is_void flag
to hide discarded items from queries. Overrides delete
methods to set flag and soft-delete instead of removing
rows from the database.
"""
from django.apps import apps
from django.contrib.admin.utils import NestedObjects
from django.db import models
from django.db.models import signals
class DeleteNotPermitted(Exception):
pass
class ObjectIsVoid(models.ObjectDoesNotExist):
pass
class AppQuerySet(models.QuerySet):
def _wrap(self, func, args, kwargs):
"""
If an object doesn't exist, check to see if it exists. If it doesn't, remove the
`is_void` filter and raise IsVoid exception if found.
"""
try:
r = func(*args, **kwargs)
except self.model.DoesNotExist, e:
for node in self.query.where.children[:]:
if hasattr(node, 'children'):
for child in node.children:
if child.lhs.target.attname == 'is_void':
node.children.remove(child)
if not node.children:
self.query.where.children.remove(node)
q = super(AppQuerySet, self).filter(*args, **kwargs)
if q.exists():
raise self.model.IsVoid('Object exists but is void.')
else:
raise e
return r
def get(self, *args, **kwargs):
return self._wrap(
super(AppQuerySet, self).get, args, kwargs
)
def get_or_create(self, *args, **kwargs):
return self._wrap(
super(AppQuerySet, self).get_or_create, args, kwargs
)
def delete(self, cascade=True, **kwargs):
if self.model.PREVENT_DELETE:
raise DeleteNotPermitted()
if cascade:
for obj in self.all():
obj.delete()
else:
return self.update(is_void=True)
class AppManager(models.Manager):
queryset_class = AppQuerySet
use_for_related_fields = True
def get_queryset(self, exclude_void=True):
q = self.queryset_class(self.model)
if hasattr(self, 'core_filters'):
q = q.filter(
**self.core_filters
)
if exclude_void:
q = q.exclude(is_void=True)
return q
def all_objects_including_void(self):
return self.get_queryset(exclude_void=False)
class AppModelMeta(type(models.Model)):
def __new__(cls, name, parents, dct):
new_class = super(AppModelMeta, cls).__new__(cls, name, parents, dct)
if hasattr(new_class, 'DoesNotExist'):
new_class.IsVoid = type('IsVoid', (new_class.DoesNotExist, ObjectIsVoid), {})
return new_class
class AppModel(models.Model):
__metaclass__ = AppModelMeta
PREVENT_DELETE = False
is_void = models.BooleanField(default=False)
objects = AppManager()
class Meta:
abstract = True
def delete(self, cascade=True, **kwargs):
if self.PREVENT_DELETE:
raise DeleteNotPermitted()
if cascade:
collector = NestedObjects(using='default')
collector.collect([self])
field_updates = collector.field_updates
for cls, to_update in field_updates.iteritems():
for (field, value), instances in to_update.iteritems():
cls.objects.filter(
pk__in={o.pk for o in instances}
).update(
**{field.attname: value}
)
for klass, objs in collector.data.iteritems():
try:
klass._meta.get_field('is_void')
except models.FieldDoesNotExist:
pass
else:
klass.objects.filter(pk__in={o.pk for o in objs}).update(
is_void=True
)
else:
self.is_void = True
self.save()
signals.post_delete.send(
sender=self.__class__, instance=self
)
@c17r
c17r commented Jun 29, 2016 edited

Read your Medium post about this. Great stuff!

Couple of questions:

  1. Shouldn't Line 56 be passing kwargs down to the object?
  2. Shouldn't Line 116 be calling delete()? Mostly in case the child object has a custom manager that subclasses AppManager and may have some other actions happen at delete. An outlier case, definitely, but still possible.
  3. Shouldn't Lines 43 and 48 be calling the super() function with *args, **kwargs instead of args, kwargs?

First sentence of Line 21 could be a little clearer: If an object "doesn't exist", check to see if it's there but voided.

@WnP
WnP commented Jun 30, 2016

@c17r

about 3. if you want to do so you need to re-write AppQuerySet._wrap signature to:

 def _wrap(self, func, *args, **kwargs):
@valentjedi

@c17r
Regarding 3. I think that calling delete() instead update will cause stack blowing by infinite call of instance's delete method. Or I miss something. May be this could be avoided by calling obj.delete(cascade=False) in line 56.
Regarding 1. I agree.

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