不要定义要按需加载列的映射。然后使用mapper
对象配置那些在Deferred Column Loading中描述的对象。此处修改代码:
from sqlalchemy.ext.declarative import declarative_base
Base = declarative_base()
class SomeClass(Base):
__tablename__ = 'some_table'
id = Column(Integer, primary_key=True)
name = Column(String(50))
#big_name = Column(String(500))
SomeClass.__table__.append_column(Column('big_name', String(500)))
SomeClass.__mapper__.add_property('big_name', deferred(SomeClass.__table__.c.big_name))
运行该测试代码:
c = session.query(SomeClass).first()
# here SQL is loading all configured properties, but big_name
print "c: ", c
# only here another SQL request is made to load the property
print "big_name: ", c.big_name
生成日志摘录:
... INFO sqlalchemy.engine.base.Engine.0x...77d0 SELECT some_table.id AS some_table_id, some_table.name AS some_table_name
FROM some_table
LIMIT 1 OFFSET 0
... INFO sqlalchemy.engine.base.Engine.0x...77d0 SELECT some_table.big_name AS some_table_big_name
FROM some_table
WHERE some_table.id = ?
来源
2010-02-18 17:47:30
van
是什么你可能想要的理由o推迟加载?您是否愿意更改模型或数据库模式以使其工作? – van 2010-02-18 16:25:21