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Fixed #2705: added a `select_for_update()` clause to querysets.

A number of people worked on this patch over the years -- Hawkeye, Colin Grady,
KBS, sakyamuni, anih, jdemoor, and Issak Kelly. Thanks to them all, and
apologies if I missed anyone.

Special thanks to Dan Fairs for picking it up again at the end and seeing this
through to commit.

git-svn-id: bcc190cf-cafb-0310-a4f2-bffc1f526a37
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jacobian committed Apr 20, 2011
1 parent 99c1794 commit 8f0f73c7b8b110489a1a127cc47e3cabb0eea646
@@ -168,6 +168,7 @@ answer newbie questions, and generally made Django that much better:
Tomáš Ehrlich <>
Dirk Eschler <>
Dan Fairs <>
Marc Fargas <>
Szilveszter Farkas <>
Grigory Fateyev <>
@@ -279,6 +279,8 @@ class BaseDatabaseFeatures(object):
# integer primary keys.
related_fields_match_type = False
allow_sliced_subqueries = True
has_select_for_update = False
has_select_for_update_nowait = False
# Does the default test database allow multiple connections?
# Usually an indication that the test database is in-memory
@@ -476,6 +478,15 @@ def force_no_ordering(self):
return []
def for_update_sql(self, nowait=False):
Returns the FOR UPDATE SQL clause to lock rows for an update operation.
if nowait:
return 'FOR UPDATE'
def fulltext_search_sql(self, field_name):
Returns the SQL WHERE clause to use in order to perform a full-text
@@ -124,6 +124,8 @@ class DatabaseFeatures(BaseDatabaseFeatures):
allows_group_by_pk = True
related_fields_match_type = True
allow_sliced_subqueries = False
has_select_for_update = True
has_select_for_update_nowait = False
supports_forward_references = False
supports_long_model_names = False
supports_microsecond_precision = False
@@ -70,6 +70,8 @@ class DatabaseFeatures(BaseDatabaseFeatures):
needs_datetime_string_cast = False
interprets_empty_strings_as_nulls = True
uses_savepoints = True
has_select_for_update = True
has_select_for_update_nowait = True
can_return_id_from_insert = True
allow_sliced_subqueries = False
supports_subqueries_in_group_by = False
@@ -70,6 +70,9 @@ class DatabaseFeatures(BaseDatabaseFeatures):
requires_rollback_on_dirty_transaction = True
has_real_datatype = True
can_defer_constraint_checks = True
has_select_for_update = True
has_select_for_update_nowait = True
class DatabaseWrapper(BaseDatabaseWrapper):
vendor = 'postgresql'
@@ -164,6 +164,9 @@ def latest(self, *args, **kwargs):
def order_by(self, *args, **kwargs):
return self.get_query_set().order_by(*args, **kwargs)
def select_for_update(self, *args, **kwargs):
return self.get_query_set().select_for_update(*args, **kwargs)
def select_related(self, *args, **kwargs):
return self.get_query_set().select_related(*args, **kwargs)
@@ -435,6 +435,7 @@ def delete(self):
del_query._for_write = True
# Disable non-supported fields.
del_query.query.select_for_update = False
del_query.query.select_related = False
@@ -583,6 +584,18 @@ def complex_filter(self, filter_obj):
return self._filter_or_exclude(None, **filter_obj)
def select_for_update(self, **kwargs):
Returns a new QuerySet instance that will select objects with a
# Default to false for nowait
nowait = kwargs.pop('nowait', False)
obj = self._clone()
obj.query.select_for_update = True
obj.query.select_for_update_nowait = nowait
return obj
def select_related(self, *fields, **kwargs):
Returns a new QuerySet instance that will select related objects.
@@ -1,11 +1,13 @@
from django.core.exceptions import FieldError
from django.db import connections
from django.db import transaction
from django.db.backends.util import truncate_name
from django.db.models.sql.constants import *
from django.db.models.sql.datastructures import EmptyResultSet
from django.db.models.sql.expressions import SQLEvaluator
from django.db.models.sql.query import get_proxied_model, get_order_dir, \
select_related_descend, Query
from django.db.utils import DatabaseError
class SQLCompiler(object):
def __init__(self, query, connection, using):
@@ -117,6 +119,14 @@ def as_sql(self, with_limits=True, with_col_aliases=False):
result.append('LIMIT %d' % val)
result.append('OFFSET %d' % self.query.low_mark)
if self.query.select_for_update and self.connection.features.has_select_for_update:
# If we've been asked for a NOWAIT query but the backend does not support it,
# raise a DatabaseError otherwise we could get an unexpected deadlock.
nowait = self.query.select_for_update_nowait
if nowait and not self.connection.features.has_select_for_update_nowait:
raise DatabaseError('NOWAIT is not supported on this database backend.')
return ' '.join(result), tuple(params)
def as_nested_sql(self):
@@ -677,6 +687,11 @@ def results_iter(self):
resolve_columns = hasattr(self, 'resolve_columns')
fields = None
has_aggregate_select = bool(self.query.aggregate_select)
# Set transaction dirty if we're using SELECT FOR UPDATE to ensure
# a subsequent commit/rollback is executed, so any database locks
# are released.
if self.query.select_for_update and transaction.is_managed(self.using):
for rows in self.execute_sql(MULTI):
for row in rows:
if resolve_columns:
@@ -125,6 +125,8 @@ def __init__(self, model, where=WhereNode):
self.order_by = []
self.low_mark, self.high_mark = 0, None # Used for offset/limit
self.distinct = False
self.select_for_update = False
self.select_for_update_nowait = False
self.select_related = False
self.related_select_cols = []
@@ -254,6 +256,8 @@ def clone(self, klass=None, memo=None, **kwargs):
obj.order_by = self.order_by[:]
obj.low_mark, obj.high_mark = self.low_mark, self.high_mark
obj.distinct = self.distinct
obj.select_for_update = self.select_for_update
obj.select_for_update_nowait = self.select_for_update_nowait
obj.select_related = self.select_related
obj.related_select_cols = []
obj.aggregates = copy.deepcopy(self.aggregates, memo=memo)
@@ -360,6 +364,7 @@ def get_aggregation(self, using):
query.select_for_update = False
query.select_related = False
query.related_select_cols = []
query.related_select_fields = []
@@ -359,6 +359,13 @@ store a timezone-aware ``time`` or ``datetime`` to a
:class:`~django.db.models.TimeField` or :class:`~django.db.models.DateTimeField`
respectively, a ``ValueError`` is raised rather than truncating data.
Row locking with ``QuerySet.select_for_update()``
MySQL does not support the ``NOWAIT`` option to the ``SELECT ... FOR UPDATE``
statement. If ``select_for_update()`` is used with ``nowait=True`` then a
``DatabaseError`` will be raised.
.. _sqlite-notes:
SQLite notes
@@ -493,6 +500,12 @@ If you're getting this error, you can solve it by:
This will simply make SQLite wait a bit longer before throwing "database
is locked" errors; it won't really do anything to solve them.
``QuerySet.select_for_update()`` not supported
SQLite does not support the ``SELECT ... FOR UPDATE`` syntax. Calling it will
have no effect.
.. _oracle-notes:
Oracle notes
@@ -966,6 +966,46 @@ For example::
# queries the database with the 'backup' alias
>>> Entry.objects.using('backup')
.. method:: select_for_update(nowait=False)
.. versionadded:: 1.4
Returns a queryset that will lock rows until the end of the transaction,
generating a ``SELECT ... FOR UPDATE`` SQL statement on supported databases.
For example::
entries = Entry.objects.select_for_update().filter(author=request.user)
All matched entries will be locked until the end of the transaction block,
meaning that other transactions will be prevented from changing or acquiring
locks on them.
Usually, if another transaction has already acquired a lock on one of the
selected rows, the query will block until the lock is released. If this is
not the behaviour you want, call ``select_for_update(nowait=True)``. This will
make the call non-blocking. If a conflicting lock is already acquired by
another transaction, ``django.db.utils.DatabaseError`` will be raised when
the queryset is evaluated.
Note that using ``select_for_update`` will cause the current transaction to be
set dirty, if under transaction management. This is to ensure that Django issues
a ``COMMIT`` or ``ROLLBACK``, releasing any locks held by the ``SELECT FOR
Currently, the ``postgresql_psycopg2``, ``oracle``, and ``mysql``
database backends support ``select_for_update()``. However, MySQL has no
support for the ``nowait`` argument.
Passing ``nowait=True`` to ``select_for_update`` using database backends that
do not support ``nowait``, such as MySQL, will cause a ``DatabaseError`` to be
raised. This is in order to prevent code unexpectedly blocking.
Using ``select_for_update`` on backends which do not support
``SELECT ... FOR UPDATE`` (such as SQLite) will have no effect.
Methods that do not return QuerySets
@@ -0,0 +1 @@
@@ -0,0 +1,4 @@
from django.db import models
class Person(models.Model):
name = models.CharField(max_length=30)
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