SQLAlchemy黄金体验

session的生命期

引用
Web Server          Web Framework        User-defined Controller Call
--------------      --------------       ------------------------------
web request    ->
                    call controller ->   # call Session().  this establishes a new,
                                         # contextual Session.
                                         session = Session()

                                         # load some objects, save some changes
                                         objects = session.query(MyClass).all()

                                         # some other code calls Session, it's the
                                         # same contextual session as "sess"
                                         session2 = Session()
                                         session2.add(foo)
                                         session2.commit()

                                         # generate content to be returned
                                         return generate_content()
                    Session.remove() <-
web response   <-

参考链接:
http://docs.sqlalchemy.org/en/latest/orm/session.html#sqlalchemy.orm.session.sessionmaker

遇到下边这个异常应该如何解决?
引用
TimeoutError: QueuePool limit of size 5 overflow 10 reached, connection timed out, timeout 30。

You are probably not removing the session after the request has been served.
Check out SQLAlchemy documentation on Session.remove.
使用完Seesion后,应该立即把它删除。
示例代码如下:
    
    def load(self, class_alias, filterExpr):
        klass = pyamf.load_class(class_alias).klass
        session = Schema().session
        result = session.query(klass).filter(filterExpr).all()
        session.remove()
        return result

参考链接: http://stackoverflow.com/questions/2314844/limiting-pyamf-flex-sessions-or-setting-a-timeout

数据库路径需要转成ascii码,否则回sqlalchemy会抛出下边的错误
sqlalchemy.exc.DBAPIError: (error) argument for 's' must be a string None None

如何在建表的时候为字段指定默认值?
使用PassiveDefault(),这是数据库级别的默认值。
如果希望默认值仅在Insert的时候使用的话,使用default
Field(Unicode(256), default=u'default name', nullable=False)


使用@hybrid_method时为啥SQLAlchemy抛出这么一个错误:sqlalchemy.exc.ArgumentError: filter() argument must be of type sqlalchemy.sql.ClauseElement or string
hybrid_method不要直接返回字符串或者布尔值,而是应该返回表达式,如:
return self.name == categoryName


过滤条件中,in_和contains的区别

引用
contains(other, **kwargs)
    Implement the ‘contains’ operator.
    In a column context, produces the clause LIKE '%<other>%'

引用
in_(other)
    Implement the in operator.
    In a column context, produces the clause a IN other. “other” may be a     tuple/list of column expressions, or a select() construct.


SQLAlchemy的children默认使用的List如何将其改为Dictionary?
通过为relationship设置collection_class属性,示例代码如下:
children = relationship("Category", lazy="joined", join_depth =1, backref = backref('parent', remote_side=[id]), collection_class=attribute_mapped_collection('name'),)


将小数保存到数据库时如何指定保存小数点后边的数字的个数?
通过设置Numeric的第2个参数,示例代码如下:
unitPrice = Column(Numeric(5, 2))

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