/usr/lib/python2.7/dist-packages/oslo_db/sqlalchemy/session.py is in python-oslo.db 4.7.0-2ubuntu1.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175 176 177 178 179 180 181 182 183 184 185 186 187 188 189 190 191 192 193 194 195 196 | # Copyright 2010 United States Government as represented by the
# Administrator of the National Aeronautics and Space Administration.
# All Rights Reserved.
#
# Licensed under the Apache License, Version 2.0 (the "License"); you may
# not use this file except in compliance with the License. You may obtain
# a copy of the License at
#
# http://www.apache.org/licenses/LICENSE-2.0
#
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
# WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
# License for the specific language governing permissions and limitations
# under the License.
"""Session Handling for SQLAlchemy backend.
Recommended ways to use sessions within this framework:
* Use the ``enginefacade`` system for connectivity, session and
transaction management:
.. code-block:: python
from oslo_db.sqlalchemy import enginefacade
@enginefacade.reader
def get_foo(context, foo):
return (model_query(models.Foo, context.session).
filter_by(foo=foo).
first())
@enginefacade.writer
def update_foo(context, id, newfoo):
(model_query(models.Foo, context.session).
filter_by(id=id).
update({'foo': newfoo}))
@enginefacade.writer
def create_foo(context, values):
foo_ref = models.Foo()
foo_ref.update(values)
foo_ref.save(context.session)
return foo_ref
In the above system, transactions are committed automatically, and
are shared among all dependent database methods. Ensure
that methods which "write" data are enclosed within @writer blocks.
.. note:: Statements in the session scope will not be automatically retried.
* If you create models within the session, they need to be added, but you
do not need to call `model.save()`:
.. code-block:: python
@enginefacade.writer
def create_many_foo(context, foos):
for foo in foos:
foo_ref = models.Foo()
foo_ref.update(foo)
context.session.add(foo_ref)
@enginefacade.writer
def update_bar(context, foo_id, newbar):
foo_ref = (model_query(models.Foo, context.session).
filter_by(id=foo_id).
first())
(model_query(models.Bar, context.session).
filter_by(id=foo_ref['bar_id']).
update({'bar': newbar}))
The two queries in `update_bar` can alternatively be expressed using
a single query, which may be more efficient depending on scenario:
.. code-block:: python
@enginefacade.writer
def update_bar(context, foo_id, newbar):
subq = (model_query(models.Foo.id, context.session).
filter_by(id=foo_id).
limit(1).
subquery())
(model_query(models.Bar, context.session).
filter_by(id=subq.as_scalar()).
update({'bar': newbar}))
For reference, this emits approximately the following SQL statement:
.. code-block:: sql
UPDATE bar SET bar = ${newbar}
WHERE id=(SELECT bar_id FROM foo WHERE id = ${foo_id} LIMIT 1);
.. note:: `create_duplicate_foo` is a trivially simple example of catching an
exception while using a savepoint. Here we create two duplicate
instances with same primary key, must catch the exception out of context
managed by a single session:
.. code-block:: python
@enginefacade.writer
def create_duplicate_foo(context):
foo1 = models.Foo()
foo2 = models.Foo()
foo1.id = foo2.id = 1
try:
with context.session.begin_nested():
session.add(foo1)
session.add(foo2)
except exception.DBDuplicateEntry as e:
handle_error(e)
* The enginefacade system eliminates the need to decide when sessions need
to be passed between methods. All methods should instead share a common
context object; the enginefacade system will maintain the transaction
across method calls.
.. code-block:: python
@enginefacade.writer
def myfunc(context, foo):
# do some database things
bar = _private_func(context, foo)
return bar
def _private_func(context, foo):
with enginefacade.using_writer(context) as session:
# do some other database things
session.add(SomeObject())
return bar
* Avoid ``with_lockmode('UPDATE')`` when possible.
FOR UPDATE is not compatible with MySQL/Galera. Instead, an "opportunistic"
approach should be used, such that if an UPDATE fails, the entire
transaction should be retried. The @wrap_db_retry decorator is one
such system that can be used to achieve this.
Enabling soft deletes:
* To use/enable soft-deletes, `SoftDeleteMixin` may be used. For example:
.. code-block:: python
class NovaBase(models.SoftDeleteMixin, models.ModelBase):
pass
Efficient use of soft deletes:
* While there is a ``model.soft_delete()`` method, prefer
``query.soft_delete()``. Some examples:
.. code-block:: python
@enginefacade.writer
def soft_delete_bar(context):
# synchronize_session=False will prevent the ORM from attempting
# to search the Session for instances matching the DELETE;
# this is typically not necessary for small operations.
count = model_query(BarModel, context.session).\\
find(some_condition).soft_delete(synchronize_session=False)
if count == 0:
raise Exception("0 entries were soft deleted")
@enginefacade.writer
def complex_soft_delete_with_synchronization_bar(context):
# use synchronize_session='evaluate' when you'd like to attempt
# to update the state of the Session to match that of the DELETE.
# This is potentially helpful if the operation is complex and
# continues to work with instances that were loaded, though
# not usually needed.
count = (model_query(BarModel, context.session).
find(some_condition).
soft_delete(synchronize_session='evaulate'))
if count == 0:
raise Exception("0 entries were soft deleted")
"""
from oslo_db.sqlalchemy import enginefacade
from oslo_db.sqlalchemy import engines
from oslo_db.sqlalchemy import orm
EngineFacade = enginefacade.LegacyEngineFacade
create_engine = engines.create_engine
get_maker = orm.get_maker
Query = orm.Query
Session = orm.Session
__all__ = ["EngineFacade", "create_engine", "get_maker", "Query", "Session"]
|