root / snf-common / synnefo / lib / db / managers.py @ 1fd14d3d
History | View | Annotate | Download (3.7 kB)
1 |
# Copyright 2012, 2013 GRNET S.A. All rights reserved.
|
---|---|
2 |
#
|
3 |
# Redistribution and use in source and binary forms, with or without
|
4 |
# modification, are permitted provided that the following conditions
|
5 |
# are met:
|
6 |
#
|
7 |
# 1. Redistributions of source code must retain the above copyright
|
8 |
# notice, this list of conditions and the following disclaimer.
|
9 |
#
|
10 |
# 2. Redistributions in binary form must reproduce the above copyright
|
11 |
# notice, this list of conditions and the following disclaimer in the
|
12 |
# documentation and/or other materials provided with the distribution.
|
13 |
#
|
14 |
# THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
|
15 |
# ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
|
16 |
# IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
|
17 |
# ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
|
18 |
# FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
|
19 |
# DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
|
20 |
# OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
|
21 |
# HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
|
22 |
# LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
|
23 |
# OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
|
24 |
# SUCH DAMAGE.
|
25 |
#
|
26 |
# The views and conclusions contained in the software and documentation are
|
27 |
# those of the authors and should not be interpreted as representing official
|
28 |
# policies, either expressed or implied, of GRNET S.A.
|
29 |
|
30 |
from django.db import connections |
31 |
from django.db.models import Manager |
32 |
from django.db.models.query import QuerySet, EmptyQuerySet |
33 |
from django.db.models.sql.datastructures import EmptyResultSet |
34 |
|
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.filter(public=True).select_for_update()
|
49 |
|
50 |
"""
|
51 |
|
52 |
def get_query_set(self): |
53 |
return ForUpdateQuerySet(self.model, using=self._db) |
54 |
|
55 |
def get_for_update(self, *args, **kwargs): |
56 |
query = for_update(self.filter(*args, **kwargs))
|
57 |
query = list(query)
|
58 |
num = len(query)
|
59 |
if num == 1: |
60 |
return query[0] |
61 |
if not num: |
62 |
raise self.model.DoesNotExist( |
63 |
"%s matching query does not exist. "
|
64 |
"Lookup parameters were %s" %
|
65 |
(self.model._meta.object_name, kwargs))
|
66 |
raise self.model.MultipleObjectsReturned( |
67 |
"get() returned more than one %s -- it returned %s! "
|
68 |
"Lookup parameters were %s" %
|
69 |
(self.model._meta.object_name, num, kwargs))
|
70 |
|
71 |
|
72 |
class ForUpdateQuerySet(QuerySet): |
73 |
|
74 |
def select_for_update(self): |
75 |
return for_update(self) |
76 |
|
77 |
|
78 |
def for_update(query): |
79 |
""" Rewrite query using SELECT .. FOR UPDATE.
|
80 |
|
81 |
"""
|
82 |
if 'sqlite' in connections[query.db].settings_dict['ENGINE'].lower(): |
83 |
# SQLite does not support FOR UPDATE
|
84 |
return query
|
85 |
try:
|
86 |
sql, params = query.query.get_compiler(query.db).as_sql() |
87 |
except EmptyResultSet:
|
88 |
return EmptyQuerySet()
|
89 |
return query.model._default_manager.raw(sql.rstrip() + ' FOR UPDATE', |
90 |
params) |