| 27 |
27 |
# those of the authors and should not be interpreted as representing official
|
| 28 |
28 |
# policies, either expressed or implied, of GRNET S.A.
|
| 29 |
29 |
|
| 30 |
|
from django.db import connections
|
| 31 |
30 |
from django.db.models import Manager
|
| 32 |
31 |
from django.db.models.query import QuerySet
|
| 33 |
32 |
|
| 34 |
33 |
|
| 35 |
|
class ForUpdateManager(Manager):
|
| 36 |
|
"""Model manager implementing SELECT .. FOR UPDATE statement
|
| 37 |
|
|
| 38 |
|
This manager implements select_for_update() method in order to use
|
| 39 |
|
row-level locking in the database and guarantee exclusive access, since
|
| 40 |
|
this method is only implemented in Django>=1.4.
|
| 41 |
|
|
| 42 |
|
Non-blocking reads are not implemented, and each query including a row
|
| 43 |
|
that is locked by another transaction will block until the lock is
|
| 44 |
|
released. Also care must be taken in order to avoid deadlocks or retry
|
| 45 |
|
transactions that abort due to deadlocks.
|
| 46 |
|
|
| 47 |
|
Example:
|
| 48 |
|
networks = Network.objects.select_for_update().filter(public=True)
|
| 49 |
|
|
| 50 |
|
"""
|
| 51 |
|
|
| 52 |
|
def select_for_update(self, *args, **kwargs):
|
| 53 |
|
return ForUpdateQuerySet(self.model, using=self.db)
|
| 54 |
|
|
| 55 |
|
|
| 56 |
|
class ForUpdateQuerySet(QuerySet):
|
| 57 |
|
"""QuerySet implmenting SELECT .. FOR UPDATE statement
|
| 58 |
|
|
| 59 |
|
This QuerySet overrides filter and get methods in order to implement
|
| 60 |
|
select_for_update() statement, by appending 'FOR UPDATE' to the end
|
| 61 |
|
for the SQL query.
|
| 62 |
|
|
| 63 |
|
"""
|
| 64 |
|
|
| 65 |
|
def filter(self, *args, **kwargs):
|
| 66 |
|
query = super(ForUpdateQuerySet, self).filter(*args, **kwargs)
|
| 67 |
|
return for_update(query)
|
| 68 |
|
|
| 69 |
|
def get(self, *args, **kwargs):
|
| 70 |
|
query = self.filter(*args, **kwargs)
|
| 71 |
|
query = list(query)
|
| 72 |
|
num = len(query)
|
| 73 |
|
if num == 1:
|
| 74 |
|
return query[0]
|
| 75 |
|
if not num:
|
| 76 |
|
raise self.model.DoesNotExist("%s matching query does not exist. "
|
| 77 |
|
"Lookup parameters were %s" %
|
| 78 |
|
(self.model._meta.object_name,
|
| 79 |
|
kwargs))
|
| 80 |
|
raise self.model.MultipleObjectsReturned(
|
| 81 |
|
"get() returned more than one %s -- it returned %s! "
|
| 82 |
|
"Lookup parameters were %s" %
|
| 83 |
|
(self.model._meta.object_name, num, kwargs))
|
| 84 |
|
|
| 85 |
|
|
| 86 |
|
def for_update(query):
|
| 87 |
|
"""Rewrite query using SELECT .. FOR UPDATE."""
|
| 88 |
|
if 'sqlite' in connections[query.db].settings_dict['ENGINE'].lower():
|
| 89 |
|
# SQLite does not support FOR UPDATE
|
| 90 |
|
return query
|
| 91 |
|
sql, params = query.query.get_compiler(query.db).as_sql()
|
| 92 |
|
return query.model._default_manager.raw(sql.rstrip() + ' FOR UPDATE',
|
| 93 |
|
params)
|
| 94 |
|
|
| 95 |
|
|
| 96 |
|
class ProtectedDeleteManager(ForUpdateManager):
|
|
34 |
class ProtectedDeleteManager(Manager):
|
| 97 |
35 |
"""Manager for protecting Backend deletion.
|
| 98 |
36 |
|
| 99 |
37 |
Call Backend delete() method in order to prevent deletion
|