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# those of the authors and should not be interpreted as representing official
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# policies, either expressed or implied, of GRNET S.A.
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from django.db import connections
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from django.db.models import Manager
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from django.db.models.query import QuerySet
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class ForUpdateManager(Manager):
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"""Model manager implementing SELECT .. FOR UPDATE statement
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This manager implements select_for_update() method in order to use
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row-level locking in the database and guarantee exclusive access, since
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this method is only implemented in Django>=1.4.
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Non-blocking reads are not implemented, and each query including a row
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that is locked by another transaction will block until the lock is
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released. Also care must be taken in order to avoid deadlocks or retry
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transactions that abort due to deadlocks.
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Example:
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networks = Network.objects.select_for_update().filter(public=True)
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"""
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def select_for_update(self, *args, **kwargs):
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return ForUpdateQuerySet(self.model, using=self.db)
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class ForUpdateQuerySet(QuerySet):
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"""QuerySet implmenting SELECT .. FOR UPDATE statement
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This QuerySet overrides filter and get methods in order to implement
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select_for_update() statement, by appending 'FOR UPDATE' to the end
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for the SQL query.
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"""
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def filter(self, *args, **kwargs):
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query = super(ForUpdateQuerySet, self).filter(*args, **kwargs)
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return for_update(query)
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def get(self, *args, **kwargs):
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query = self.filter(*args, **kwargs)
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query = list(query)
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num = len(query)
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if num == 1:
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return query[0]
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if not num:
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raise self.model.DoesNotExist("%s matching query does not exist. "
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"Lookup parameters were %s" %
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(self.model._meta.object_name,
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kwargs))
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raise self.model.MultipleObjectsReturned(
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"get() returned more than one %s -- it returned %s! "
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"Lookup parameters were %s" %
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(self.model._meta.object_name, num, kwargs))
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def for_update(query):
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"""Rewrite query using SELECT .. FOR UPDATE."""
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if 'sqlite' in connections[query.db].settings_dict['ENGINE'].lower():
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# SQLite does not support FOR UPDATE
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return query
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sql, params = query.query.get_compiler(query.db).as_sql()
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return query.model._default_manager.raw(sql.rstrip() + ' FOR UPDATE',
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params)
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class ProtectedDeleteManager(ForUpdateManager):
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class ProtectedDeleteManager(Manager):
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"""Manager for protecting Backend deletion.
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Call Backend delete() method in order to prevent deletion
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