# Django 3.2.12 documentation

# Source code for django.db.models.deletion

    import operator
    from collections import Counter, defaultdict
    from functools import partial, reduce
    from itertools import chain
    from operator import attrgetter

    from django.db import IntegrityError, connections, transaction
    from django.db.models import query_utils, signals, sql

    class ProtectedError(IntegrityError):
        def __init__(self, msg, protected_objects):
            self.protected_objects = protected_objects
            super().__init__(msg, protected_objects)

    class RestrictedError(IntegrityError):
        def __init__(self, msg, restricted_objects):
            self.restricted_objects = restricted_objects
            super().__init__(msg, restricted_objects)

    def CASCADE(collector, field, sub_objs, using):
        collector.collect(
            sub_objs, source=field.remote_field.model, source_attr=field.name,
            nullable=field.null, fail_on_restricted=False,
        )
        if field.null and not connections[using].features.can_defer_constraint_checks:
            collector.add_field_update(field, None, sub_objs)

    def PROTECT(collector, field, sub_objs, using):
        raise ProtectedError(
            "Cannot delete some instances of model '%s' because they are "
            "referenced through a protected foreign key: '%s.%s'" % (
                field.remote_field.model.__name__, sub_objs[0].__class__.__name__, field.name
            ),
            sub_objs
        )

    def RESTRICT(collector, field, sub_objs, using):
        collector.add_restricted_objects(field, sub_objs)
        collector.add_dependency(field.remote_field.model, field.model)

    def SET(value):
        if callable(value):
            def set_on_delete(collector, field, sub_objs, using):
                collector.add_field_update(field, value(), sub_objs)
        else:
            def set_on_delete(collector, field, sub_objs, using):
                collector.add_field_update(field, value, sub_objs)
        set_on_delete.deconstruct = lambda: ('django.db.models.SET', (value,), {})
        return set_on_delete

    def SET_NULL(collector, field, sub_objs, using):
        collector.add_field_update(field, None, sub_objs)

    def SET_DEFAULT(collector, field, sub_objs, using):
        collector.add_field_update(field, field.get_default(), sub_objs)

    def DO_NOTHING(collector, field, sub_objs, using):
        pass

    def get_candidate_relations_to_delete(opts):
        # The candidate relations are the ones that come from N-1 and 1-1 relations.
        # N-N  (i.e., many-to-many) relations aren't candidates for deletion.
        return (
            f for f in opts.get_fields(include_hidden=True)
            if f.auto_created and not f.concrete and (f.one_to_one or f.one_to_many)
        )

    class Collector:
        def __init__(self, using):
            self.using = using
            # Initially, {model: {instances}}, later values become lists.
            self.data = defaultdict(set)
            # {model: {(field, value): {instances}}}
            self.field_updates = defaultdict(partial(defaultdict, set))
            # {model: {field: {instances}}}
            self.restricted_objects = defaultdict(partial(defaultdict, set))
            # fast_deletes is a list of queryset-likes that can be deleted without
            # fetching the objects into memory.
            self.fast_deletes = []

            # Tracks deletion-order dependency for databases without transactions
            # or ability to defer constraint checks. Only concrete model classes
            # should be included, as the dependencies exist only between actual
            # database tables; proxy models are represented here by their concrete
            # parent.
            self.dependencies = defaultdict(set)  # {model: {models}}

        def add(self, objs, source=None, nullable=False, reverse_dependency=False):
            """
            Add 'objs' to the collection of objects to be deleted.  If the call is
            the result of a cascade, 'source' should be the model that caused it,
            and 'nullable' should be set to True if the relation can be null.

            Return a list of all objects that were not already collected.
            """
            if not objs:
                return []
            new_objs = []
            model = objs[0].__class__
            instances = self.data[model]
            for obj in objs:
                if obj not in instances:
                    new_objs.append(obj)
            instances.update(new_objs)
            # Nullable relationships can be ignored -- they are nulled out before
            # deleting, and therefore do not affect the order in which objects have
            # to be deleted.
            if source is not None and not nullable:
                self.add_dependency(source, model, reverse_dependency=reverse_dependency)
            return new_objs

        def add_dependency(self, model, dependency, reverse_dependency=False):
            if reverse_dependency:
                model, dependency = dependency, model
            self.dependencies[model._meta.concrete_model].add(dependency._meta.concrete_model)
            self.data.setdefault(dependency, self.data.default_factory())

        def add_field_update(self, field, value, objs):
            """
            Schedule a field update. 'objs' must be a homogeneous iterable
            collection of model instances (e.g. a QuerySet).
            """
            if not objs:
                return
            model = objs[0].__class__
            self.field_updates[model][field, value].update(objs)

        def add_restricted_objects(self, field, objs):
            if objs:
                model = objs[0].__class__
                self.restricted_objects[model][field].update(objs)

        def clear_restricted_objects_from_set(self, model, objs):
            if model in self.restricted_objects:
                self.restricted_objects[model] = {
                    field: items - objs
                    for field, items in self.restricted_objects[model].items()
                }

        def clear_restricted_objects_from_queryset(self, model, qs):
            if model in self.restricted_objects:
                objs = set(qs.filter(pk__in=[
                    obj.pk
                    for objs in self.restricted_objects[model].values() for obj in objs
                ]))
                self.clear_restricted_objects_from_set(model, objs)

        def _has_signal_listeners(self, model):
            return (
                signals.pre_delete.has_listeners(model) or
                signals.post_delete.has_listeners(model)
            )

        def can_fast_delete(self, objs, from_field=None):
            """
            Determine if the objects in the given queryset-like or single object
            can be fast-deleted. This can be done if there are no cascades, no
            parents and no signal listeners for the object class.

            The 'from_field' tells where we are coming from - we need this to
            determine if the objects are in fact to be deleted. Allow also
            skipping parent -> child -> parent chain preventing fast delete of
            the child.
            """
            if from_field and from_field.remote_field.on_delete is not CASCADE:
                return False
            if hasattr(objs, '_meta'):
                model = objs._meta.model
            elif hasattr(objs, 'model') and hasattr(objs, '_raw_delete'):
                model = objs.model
            else:
                return False
            if self._has_signal_listeners(model):
                return False
            # The use of from_field comes from the need to avoid cascade back to
            # parent when parent delete is cascading to child.
            opts = model._meta
            return (
                all(link == from_field for link in opts.concrete_model._meta.parents.values()) and
                # Foreign keys pointing to this model.
                all(
                    related.field.remote_field.on_delete is DO_NOTHING
                    for related in get_candidate_relations_to_delete(opts)
                ) and (
                    # Something like generic foreign key.
                    not any(hasattr(field, 'bulk_related_objects') for field in opts.private_fields)
                )
            )

        def get_del_batches(self, objs, fields):
            """
            Return the objs in suitably sized batches for the used connection.
            """
            field_names = [field.name for field in fields]
            conn_batch_size = max(
                connections[self.using].ops.bulk_batch_size(field_names, objs), 1)
            if len(objs) > conn_batch_size:
                return [objs[i:i + conn_batch_size]
                        for i in range(0, len(objs), conn_batch_size)]
            else:
                return [objs]

        def collect(self, objs, source=None, nullable=False, collect_related=True,
                    source_attr=None, reverse_dependency=False, keep_parents=False,
                    fail_on_restricted=True):
            """
            Add 'objs' to the collection of objects to be deleted as well as all
            parent instances.  'objs' must be a homogeneous iterable collection of
            model instances (e.g. a QuerySet).  If 'collect_related' is True,
            related objects will be handled by their respective on_delete handler.

            If the call is the result of a cascade, 'source' should be the model
            that caused it and 'nullable' should be set to True, if the relation
            can be null.

            If 'reverse_dependency' is True, 'source' will be deleted before the
            current model, rather than after. (Needed for cascading to parent
            models, the one case in which the cascade follows the forwards
            direction of an FK rather than the reverse direction.)

            If 'keep_parents' is True, data of parent model's will be not deleted.

            If 'fail_on_restricted' is False, error won't be raised even if it's
            prohibited to delete such objects due to RESTRICT, that defers
            restricted object checking in recursive calls where the top-level call
            may need to collect more objects to determine whether restricted ones
            can be deleted.
            """
            if self.can_fast_delete(objs):
                self.fast_deletes.append(objs)
                return
            new_objs = self.add(objs, source, nullable,
                                reverse_dependency=reverse_dependency)
            if not new_objs:
                return

            model = new_objs[0].__class__

            if not keep_parents:
                # Recursively collect concrete model's parent models, but not their
                # related objects. These will be found by meta.get_fields()
                concrete_model = model._meta.concrete_model
                for ptr in concrete_model._meta.parents.values():
                    if ptr:
                        parent_objs = [getattr(obj, ptr.name) for obj in new_objs]
                        self.collect(parent_objs, source=model,
                                     source_attr=ptr.remote_field.related_name,
                                     collect_related=False,
                                     reverse_dependency=True,
                                     fail_on_restricted=False)
            if not collect_related:
                return

            if keep_parents:
                parents = set(model._meta.get_parent_list())
            model_fast_deletes = defaultdict(list)
            protected_objects = defaultdict(list)
            for related in get_candidate_relations_to_delete(model._meta):
                # Preserve parent reverse relationships if keep_parents=True.
                if keep_parents and related.model in parents:
                    continue
                field = related.field
                if field.remote_field.on_delete == DO_NOTHING:
                    continue
                related_model = related.related_model
                if self.can_fast_delete(related_model, from_field=field):
                    model_fast_deletes[related_model].append(field)
                    continue
                batches = self.get_del_batches(new_objs, [field])
                for batch in batches:
                    sub_objs = self.related_objects(related_model, [field], batch)
                    # Non-referenced fields can be deferred if no signal receivers
                    # are connected for the related model as they'll never be
                    # exposed to the user. Skip field deferring when some
                    # relationships are select_related as interactions between both
                    # features are hard to get right. This should only happen in
                    # the rare cases where .related_objects is overridden anyway.
                    if not (sub_objs.query.select_related or self._has_signal_listeners(related_model)):
                        referenced_fields = set(chain.from_iterable(
                            (rf.attname for rf in rel.field.foreign_related_fields)
                            for rel in get_candidate_relations_to_delete(related_model._meta)
                        ))
                        sub_objs = sub_objs.only(*tuple(referenced_fields))
                    if sub_objs:
                        try:
                            field.remote_field.on_delete(self, field, sub_objs, self.using)
                        except ProtectedError as error:
                            key = "'%s.%s'" % (field.model.__name__, field.name)
                            protected_objects[key] += error.protected_objects
            if protected_objects:
                raise ProtectedError(
                    'Cannot delete some instances of model %r because they are '
                    'referenced through protected foreign keys: %s.' % (
                        model.__name__,
                        ', '.join(protected_objects),
                    ),
                    set(chain.from_iterable(protected_objects.values())),
                )
            for related_model, related_fields in model_fast_deletes.items():
                batches = self.get_del_batches(new_objs, related_fields)
                for batch in batches:
                    sub_objs = self.related_objects(related_model, related_fields, batch)
                    self.fast_deletes.append(sub_objs)
            for field in model._meta.private_fields:
                if hasattr(field, 'bulk_related_objects'):
                    # It's something like generic foreign key.
                    sub_objs = field.bulk_related_objects(new_objs, self.using)
                    self.collect(sub_objs, source=model, nullable=True, fail_on_restricted=False)

            if fail_on_restricted:
                # Raise an error if collected restricted objects (RESTRICT) aren't
                # candidates for deletion also collected via CASCADE.
                for related_model, instances in self.data.items():
                    self.clear_restricted_objects_from_set(related_model, instances)
                for qs in self.fast_deletes:
                    self.clear_restricted_objects_from_queryset(qs.model, qs)
                if self.restricted_objects.values():
                    restricted_objects = defaultdict(list)
                    for related_model, fields in self.restricted_objects.items():
                        for field, objs in fields.items():
                            if objs:
                                key = "'%s.%s'" % (related_model.__name__, field.name)
                                restricted_objects[key] += objs
                    if restricted_objects:
                        raise RestrictedError(
                            'Cannot delete some instances of model %r because '
                            'they are referenced through restricted foreign keys: '
                            '%s.' % (
                                model.__name__,
                                ', '.join(restricted_objects),
                            ),
                            set(chain.from_iterable(restricted_objects.values())),
                        )

        def related_objects(self, related_model, related_fields, objs):
            """
            Get a QuerySet of the related model to objs via related fields.
            """
            predicate = reduce(operator.or_, (
                query_utils.Q(**{'%s__in' % related_field.name: objs})
                for related_field in related_fields
            ))
            return related_model._base_manager.using(self.using).filter(predicate)

        def instances_with_model(self):
            for model, instances in self.data.items():
                for obj in instances:
                    yield model, obj

        def sort(self):
            sorted_models = []
            concrete_models = set()
            models = list(self.data)
            while len(sorted_models) < len(models):
                found = False
                for model in models:
                    if model in sorted_models:
                        continue
                    dependencies = self.dependencies.get(model._meta.concrete_model)
                    if not (dependencies and dependencies.difference(concrete_models)):
                        sorted_models.append(model)
                        concrete_models.add(model._meta.concrete_model)
                        found = True
                if not found:
                    return
            self.data = {model: self.data[model] for model in sorted_models}

        def delete(self):
            # sort instance collections
            for model, instances in self.data.items():
                self.data[model] = sorted(instances, key=attrgetter("pk"))

            # if possible, bring the models in an order suitable for databases that
            # don't support transactions or cannot defer constraint checks until the
            # end of a transaction.
            self.sort()
            # number of objects deleted for each model label
            deleted_counter = Counter()

            # Optimize for the case with a single obj and no dependencies
            if len(self.data) == 1 and len(instances) == 1:
                instance = list(instances)[0]
                if self.can_fast_delete(instance):
                    with transaction.mark_for_rollback_on_error(self.using):
                        count = sql.DeleteQuery(model).delete_batch([instance.pk], self.using)
                    setattr(instance, model._meta.pk.attname, None)
                    return count, {model._meta.label: count}

            with transaction.atomic(using=self.using, savepoint=False):
                # send pre_delete signals
                for model, obj in self.instances_with_model():
                    if not model._meta.auto_created:
                        signals.pre_delete.send(
                            sender=model, instance=obj, using=self.using
                        )

                # fast deletes
                for qs in self.fast_deletes:
                    count = qs._raw_delete(using=self.using)
                    if count:
                        deleted_counter[qs.model._meta.label] += count

                # update fields
                for model, instances_for_fieldvalues in self.field_updates.items():
                    for (field, value), instances in instances_for_fieldvalues.items():
                        query = sql.UpdateQuery(model)
                        query.update_batch([obj.pk for obj in instances],
                                           {field.name: value}, self.using)

                # reverse instance collections
                for instances in self.data.values():
                    instances.reverse()

                # delete instances
                for model, instances in self.data.items():
                    query = sql.DeleteQuery(model)
                    pk_list = [obj.pk for obj in instances]
                    count = query.delete_batch(pk_list, self.using)
                    if count:
                        deleted_counter[model._meta.label] += count

                    if not model._meta.auto_created:
                        for obj in instances:
                            signals.post_delete.send(
                                sender=model, instance=obj, using=self.using
                            )

            # update collected instances
            for instances_for_fieldvalues in self.field_updates.values():
                for (field, value), instances in instances_for_fieldvalues.items():
                    for obj in instances:
                        setattr(obj, field.attname, value)
            for model, instances in self.data.items():
                for instance in instances:
                    setattr(instance, model._meta.pk.attname, None)
            return sum(deleted_counter.values()), dict(deleted_counter)