sqlalchemy 實現select for update

sqlalchemy 對於行級鎖有兩種實現方式，with_lockmode(self, mode): 和 with_for_update(self, read=False, nowait=False, of=None)，前者在sqlalchemy 0.9.0 被廢棄，用后者代替。所以我們使用with_for_update !

看下函數的定義：

 @_generative()
    def with_for_update(self, read=False, nowait=False, of=None):
        """return a new :class:`.Query` with the specified options for the
        ``FOR UPDATE`` clause.

        The behavior of this method is identical to that of
        :meth:`.SelectBase.with_for_update`.  When called with no arguments,
        the resulting ``SELECT`` statement will have a ``FOR UPDATE`` clause
        appended.  When additional arguments are specified, backend-specific
        options such as ``FOR UPDATE NOWAIT`` or ``LOCK IN SHARE MODE``
        can take effect.

        E.g.::

            q = sess.query(User).with_for_update(nowait=True, of=User)

        The above query on a Postgresql backend will render like::

            SELECT users.id AS users_id FROM users FOR UPDATE OF users NOWAIT

        .. versionadded:: 0.9.0 :meth:`.Query.with_for_update` supersedes
           the :meth:`.Query.with_lockmode` method.

        .. seealso::

            :meth:`.GenerativeSelect.with_for_update` - Core level method with
            full argument and behavioral description.

        """
        
read
    是標識加互斥鎖還是共享鎖. 當為 True 時, 即 for share 的語句, 是共享鎖. 多個事務可以獲取共享鎖, 互斥鎖只能一個事務獲取. 有"多個地方"都希望是"這段時間我獲取的數據不能被修改, 我也不會改", 那么只能使用共享鎖. 
nowait
    其它事務碰到鎖, 是否不等待直接"報錯". 
of
    指明上鎖的表, 如果不指明, 則查詢中涉及的所有表(行)都會加鎖.

　　

q = sess.query(User).with_for_update(nowait=True, of=User)
對應於sql：
SELECT users.id AS users_id FROM users FOR UPDATE OF users NOWAIT

 

 

 mysql 不支持這幾個參數，轉成sql都是:
SELECT users.id AS users_id FROM users FOR UPDATE

范例：

def query_city_for_update():
    session = get_session()
    with session.begin():
        query = session.query(City).with_for_update().filter(City.ID == 8)
        print 'SQL : %s' % str(query)
        print_city_info(query.first())

結果：

SQL : SELECT city."ID" AS "city_ID", city."Name" AS "city_Name", city."CountryCode" AS "city_CountryCode", city."District" AS "city_District", city."Population" AS "city_Population" 
FROM city 
WHERE city."ID" = :ID_1 FOR UPDATE

{'city': {'population': 234323, 'district': u'Utrecht', 'id': 8, 'country_code': u'NLD', 'name': u'Utrecht'}}

SELECT ... FOR UPDATE 的用法，不過鎖定(Lock)的數據是判別就得要注意一下了。由於InnoDB 預設是Row-Level Lock，所以只有「明確」的指定主鍵，MySQL 才會執行Row lock (只鎖住被選取的數據) ，否則mysql 將會執行Table Lock (將整個數據表單給鎖住)。

作者：笨手笨腳越，鏈接：https://www.jianshu.com/p/7e4de9ab942c