Python 如何根据SqlAlchemy中的表B选择表A中的一个?
我有两个表项和项描述Python 如何根据SqlAlchemy中的表B选择表A中的一个?,python,sqlalchemy,Python,Sqlalchemy,我有两个表项和项描述 class Item(Base): __tablename__ = "item" id = Column(Integer, primary_key=True) description_id = Column(Integer, ForeignKey('item_description.id')) class ItemDescription(Base): __tablename__ = "item_description" id = Col
class Item(Base):
__tablename__ = "item"
id = Column(Integer, primary_key=True)
description_id = Column(Integer, ForeignKey('item_description.id'))
class ItemDescription(Base):
__tablename__ = "item_description"
id = Column(Integer, primary_key=True)
给定一个ItemDescription列表,我想要一个项目列表,这样每个ItemDescription id都有一个项目。我不在乎哪个项目
[为清晰起见进行了编辑]
鉴于此项目和说明列表:
Item, Description
1 , 1
2 , 1
3 , 1
4 , 2
5 , 2
6 , 3
7 , 3
8 , 3
我需要一个查询,该查询将返回以下内容:
Item, Description
2 , 1
4 , 2
7 , 3
我在解决子查询等问题时遇到困难
谢谢你的帮助
from sqlalchemy.orm import relationship, backref
class Item(Base):
__tablename__ = "item"
id = Column(Integer, primary_key=True)
description_id = Column(Integer, ForeignKey('item_description.id'))
desc = relationship(User, backref=backref('desc', order_by=id))
class ItemDescription(Base):
__tablename__ = "item_description"
id = Column(Integer, primary_key=True)
Now your every ItemDescription class will have an backref called `desc` which is nothing but a list of Item.
Now you can do something like this
item_desc = session.query(ItemDescription).\
... options(joinedload('desc').all()
for item in item_desc:
print item.desc
我想这可能不会给你确切的答案。这将有助于我猜我是column_property的超级粉丝。下面是一种使用column_属性执行所需操作的方法:
from sqlalchemy import *
from sqlalchemy.orm import *
from sqlalchemy.ext.declarative import declarative_base
Base = declarative_base()
class Item(Base):
__tablename__ = 'item'
id = Column(Integer, primary_key=True)
description_id = Column(Integer, ForeignKey('item_description.id'))
class ItemDescription(Base):
__tablename__ = 'item_description'
id = Column(Integer, primary_key=True)
any_item_id = column_property(
select(
[Item.id],
id == Item.description_id,
limit=1,
).label('any_item_id'),
deferred=True,
)
e = create_engine('sqlite://', echo=True)
Base.metadata.create_all(e)
s = Session(e)
descriptions = [
ItemDescription(id=1),
ItemDescription(id=2),
ItemDescription(id=3),
]
s.add_all(descriptions)
items = [
Item(id=1, description_id=1),
Item(id=2, description_id=1),
Item(id=3, description_id=1),
Item(id=4, description_id=2),
Item(id=5, description_id=2),
Item(id=6, description_id=3),
Item(id=7, description_id=3),
Item(id=8, description_id=3),
]
s.add_all(items)
query = s.query(ItemDescription).options(undefer('any_item_id'))
for description in query:
print description.any_item_id, description.id
# alternative way without using column_property
query = s.query(
select(
[Item.id],
ItemDescription.id == Item.description_id,
limit=1,
).label('any_item_id'),
ItemDescription,
)
for item_id, description in query:
print item_id, description.id
对不起,我的解释不是很清楚,我会编辑它使其更清楚。列_属性看起来很棒,谢谢!如果我想作为单个查询而不是列属性执行此操作,我会将select放在子查询中吗?是的,您可以将select移动到查询中。我已经在答案中添加了这一点。就个人而言,我仍然更喜欢column_属性:)我喜欢column_查询方法,但也感谢您展示了in查询方式。