# Τεκμηρίωση του Django - 1.10.7

# Πηγαίος κώδικας για το django.db.models.deletion

    from collections import Counter, OrderedDict
    from operator import attrgetter

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

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

    def CASCADE(collector, field, sub_objs, using):
        collector.collect(sub_objs, source=field.remote_field.model,
                          source_attr=field.name, nullable=field.null)
        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 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(object):
        def __init__(self, using):
            self.using = using
            # Initially, {model: {instances}}, later values become lists.
            self.data = OrderedDict()
            self.field_updates = {}  # {model: {(field, value): {instances}}}
            # 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 = {}  # {model: {models}}

        def add(self, objs, source=None, nullable=False, reverse_dependency=False):
            """
            Adds '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.

            Returns a list of all objects that were not already collected.
            """
            if not objs:
                return []
            new_objs = []
            model = objs[0].__class__
            instances = self.data.setdefault(model, set())
            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:
                if reverse_dependency:
                    source, model = model, source
                self.dependencies.setdefault(
                    source._meta.concrete_model, set()).add(model._meta.concrete_model)
            return new_objs

        def add_field_update(self, field, value, objs):
            """
            Schedules 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.setdefault(
                model, {}).setdefault(
                (field, value), set()).update(objs)

        def can_fast_delete(self, objs, from_field=None):
            """
            Determines if the objects in the given queryset-like 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. Allows 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 not (hasattr(objs, 'model') and hasattr(objs, '_raw_delete')):
                return False
            model = objs.model
            if (signals.pre_delete.has_listeners(model) or
                    signals.post_delete.has_listeners(model) or
                    signals.m2m_changed.has_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
            if any(link != from_field for link in opts.concrete_model._meta.parents.values()):
                return False
            # Foreign keys pointing to this model, both from m2m and other
            # models.
            for related in get_candidate_relations_to_delete(opts):
                if related.field.remote_field.on_delete is not DO_NOTHING:
                    return False
            for field in model._meta.private_fields:
                if hasattr(field, 'bulk_related_objects'):
                    # It's something like generic foreign key.
                    return False
            return True

        def get_del_batches(self, objs, field):
            """
            Returns the objs in suitably sized batches for the used connection.
            """
            conn_batch_size = max(
                connections[self.using].ops.bulk_batch_size([field.name], 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):
            """
            Adds '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 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 six.itervalues(concrete_model._meta.parents):
                    if ptr:
                        # FIXME: This seems to be buggy and execute a query for each
                        # parent object fetch. We have the parent data in the obj,
                        # but we don't have a nice way to turn that data into parent
                        # object instance.
                        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)
            if collect_related:
                parents = model._meta.parents
                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
                    batches = self.get_del_batches(new_objs, field)
                    for batch in batches:
                        sub_objs = self.related_objects(related, batch)
                        if self.can_fast_delete(sub_objs, from_field=field):
                            self.fast_deletes.append(sub_objs)
                        elif sub_objs:
                            field.remote_field.on_delete(self, field, sub_objs, self.using)
                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)

        def related_objects(self, related, objs):
            """
            Gets a QuerySet of objects related to ``objs`` via the relation ``related``.
            """
            return related.related_model._base_manager.using(self.using).filter(
                **{"%s__in" % related.field.name: objs}
            )

        def instances_with_model(self):
            for model, instances in six.iteritems(self.data):
                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 = OrderedDict((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()

            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)
                    deleted_counter[qs.model._meta.label] += count

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

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

                # delete instances
                for model, instances in six.iteritems(self.data):
                    query = sql.DeleteQuery(model)
                    pk_list = [obj.pk for obj in instances]
                    count = query.delete_batch(pk_list, self.using)
                    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 model, instances_for_fieldvalues in six.iteritems(self.field_updates):
                for (field, value), instances in six.iteritems(instances_for_fieldvalues):
                    for obj in instances:
                        setattr(obj, field.attname, value)
            for model, instances in six.iteritems(self.data):
                for instance in instances:
                    setattr(instance, model._meta.pk.attname, None)
            return sum(deleted_counter.values()), dict(deleted_counter)