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from django.db import connections
from django.db.models import Manager
from django.db.models.query import QuerySet


class ForUpdateManager(Manager):
    """Model manager implementing SELECT .. FOR UPDATE statement

    This manager implements select_for_update() method in order to use
    row-level locking in the database and guarantee exclusive access, since
    this method is only implemented in Django>=1.4.

    Non-blocking reads are not implemented, and each query including a row
    that is locked by another transaction will block until the lock is
    released. Also care must be taken in order to avoid deadlocks or retry
    transactions that abort due to deadlocks.

    Example:
        networks = Network.objects.select_for_update().filter(public=True)

    """

    def select_for_update(self, *args, **kwargs):
        return ForUpdateQuerySet(self.model, using=self.db)


class ForUpdateQuerySet(QuerySet):
    """QuerySet implmenting SELECT .. FOR UPDATE statement

    This QuerySet overrides filter and get methods in order to implement
    select_for_update() statement, by appending 'FOR UPDATE' to the end
    for the SQL query.

    """

    def filter(self, *args, **kwargs):
        query = super(ForUpdateQuerySet, self).filter(*args, **kwargs)
        return for_update(query)

    def get(self, *args, **kwargs):
        query = self.filter(*args, **kwargs)
        query = list(query)
        num = len(query)
        if num == 1:
            return query[0]
        if not num:
            raise self.model.DoesNotExist("%s matching query does not exist. "
                                          "Lookup parameters were %s" %
                                          (self.model._meta.object_name,
                                           kwargs))
        raise self.model.MultipleObjectsReturned(
            "get() returned more than one %s -- it returned %s! "
            "Lookup parameters were %s" %
            (self.model._meta.object_name, num, kwargs))


def for_update(query):
    """Rewrite query using SELECT .. FOR UPDATE."""
    if 'sqlite' in connections[query.db].settings_dict['ENGINE'].lower():
        # SQLite  does not support FOR UPDATE
        return query
    sql, params = query.query.get_compiler(query.db).as_sql()
    return query.model._default_manager.raw(sql.rstrip() + ' FOR UPDATE',
                                            params)


class ProtectedDeleteManager(ForUpdateManager):
    """Manager for protecting Backend deletion.

    Call Backend delete() method in order to prevent deletion
    of Backends that host non-deleted VirtualMachines.

    """

    def get_query_set(self):
        return BackendQuerySet(self.model, using=self._db)


class BackendQuerySet(QuerySet):
    def delete(self):
        for backend in self._clone():
            backend.delete()