SQLAlchemy - 从自动加载表的内连接映射列的子集

Question

我刚开始使用SQLAlchemy来处理一个已有的Postgres数据库。我想做的是自动加载两个表，进行内连接，并将部分列映射到一个对象上。为了完成除了最后一部分，我写了以下代码：

from sqlalchemy.orm import mapper, sessionmaker
from sqlalchemy.sql import join, select

engine = create_engine("postgresql://<username>:@localhost/<DBname>")
metadata = Metadata(engine)
Session = sessionmaker(engine)
profiles = Table('userprofile',metadata,autoload=True)
profilefields = Table('profilefield',metadata,autoload=True)
class DBObj(object):
     pass
j = join(profiles,profilefields,profiles.c.fieldid==profilefields.c.fieldid)
mapper(DBObj,j,properties={'fieldid':[profiles.c.fieldid,profilefields.c.fieldid]})
q = session.query(DBObj).all()

我应该如何修改这段代码，以确保我只想将一些指定的列映射到DBObj上？我到处搜索，但似乎找不到实现这个的例子。

Answer 1

你需要把一个 select() 和你想要的列进行关联。使用 join() 的话，sqlalchemy 会把它变成对应的 select * from <join> 查询：

（跳过这一部分：这只是创建测试环境）

>>> from sqlalchemy.orm import mapper, sessionmaker
>>> from sqlalchemy.sql import join, select
>>> from sqlalchemy import *
>>> # engine = create_engine("postgresql://<username>:@localhost/<DBname>")
... engine = create_engine("sqlite:///:memory:")
>>> engine.execute(r"""
...     CREATE TABLE userprofile (
...         id integer primary key,
...         fieldid integer,
...         keep integer,
...         discard integer
...     )
... """)
<sqlalchemy.engine.base.ResultProxy object at 0x2390d90>
>>> engine.execute(r"""
...     CREATE TABLE profilefield (
...         id integer primary key,
...         fieldid integer,
...         keep integer,
...         discard integer
...     )
... """)
<sqlalchemy.engine.base.ResultProxy object at 0x2390e90>
>>> metadata = MetaData(engine)
>>> Session = sessionmaker(engine)
>>> profiles = Table('userprofile',metadata,autoload=True)
>>> profilefields = Table('profilefield',metadata,autoload=True)
>>> engine.execute(profiles.insert({'fieldid': 1, 'keep': 2, 'discard': 3}))
<sqlalchemy.engine.base.ResultProxy object at 0x23e9750>
>>> engine.execute(profilefields.insert({'fieldid': 1, 'keep': 2, 'discard': 3}))
<sqlalchemy.engine.base.ResultProxy object at 0x23e98d0>
>>> class DBObj(object):
...      def __repr__(self):
...         return "DBobj" + str(self.__dict__)
... 

>>> j = join(profiles,
...          profilefields,
...          profiles.c.fieldid==profilefields.c.fieldid)

下面是新内容：

>>> js = select([profiles.c.id,
...              profiles.c.keep,
...              profilefields.c.keep,
...              profiles.c.fieldid,
...              profilefields.c.fieldid],
...             use_labels=True,
...             from_obj=j)
>>> mapper(DBObj,alias(js),properties={'fieldid':[profiles.c.fieldid,profilefields.c.fieldid]})
<Mapper at 0x23e9ad0; DBObj>
>>> q = Session().query(DBObj).all()
>>> q
[DBobj{u'userprofile_keep': 2, u'userprofile_id': 1, '_sa_instance_state': <sqlalchemy.orm.state.InstanceState object at 0x23f08d0>, 'fieldid': 1, u'profilefield_keep': 2}]