Question

这是架构：

post_tag = Table("post_tag", Base.metadata,
                 Column("post_id", Integer, ForeignKey("post.id")),
                 Column("tag_id ", Integer, ForeignKey("tag.id")))


class Post(Base):
    id = Column(Integer, primary_key=True)
    tags = relationship("Tag", secondary=post_tag, backref="post", cascade="all")
    collection_id = Column(Integer, ForeignKey("collection.id"))


class Tag(Base):
    id = Column(Integer, primary_key=True)
    description = Column("description", UnicodeText, nullable=False, default="")
    post_id = Column(Integer, ForeignKey("post.id"))


class Collection(Base):
    id = Column(Integer, primary_key=True)
    title = Column(Unicode(128), nullable=False) 
    posts = relationship("Post", backref="collection", cascade="all,delete-orphan")

    tags = column_property(select([Tag])
                           .where(and_(Post.collection_id == id, Tag.post_id == Post.id))
                           .correlate_except(Tag))

基本上，Post到Tag是多对多，Collection到Post是一对多。

我希望Collection.tags在集合中返回一组不同的帖子标记。

但是，当我访问Collection.tags时，我收到以下错误：

sqlalchemy.exc.OperationalError：（sqlite3.OperationalError）只有一个结果允许作为表达式一部分的SELECT

修改

它生成的SQL

SELECT (SELECT tag.id, tag.description, tag.post_id 
FROM tag, post 
WHERE post.collection_id = collection.id AND tag.post_id = post.id) AS anon_1, collection.id AS collection_id, collection.title AS collection_title 
FROM collection 
WHERE collection.id = 1

我认为post_id = Column(Integer, ForeignKey("post.id"))错误，因为post_id位于post_tag中。但是，如果我将其更改为post_tag.post_id，则会抛出AttributeError: 'Table' object has no attribute 'post_id'

EDIT2

我将其更改为

tags = column_property(select([Tag])
                       .where(and_(Post.collection_id == id, post_tag.c.post_id == Post.id,
                                   post_tag.c.tag_id == Tag.id)))

虽然这有效

SELECT tag.id, tag.description, tag.category_id, tag.post_id 
FROM tag, post, post_tag 
WHERE post.collection_id = 1 AND post_tag.post_id = post.id AND post_tag.tag_id = tag.id

但是SQLAlchemy生成的查询不是

SELECT (SELECT tag.id, tag.description, tag.category_id, tag.post_id 
FROM tag, post, post_tag 
WHERE post.collection_id = collection.id AND post_tag.post_id = post.id AND post_tag.tag_id = tag.id) AS anon_1
FROM collection 
WHERE collection.id = 1

Answer 1

而不是column_property()，您需要relationship() with a composite "secondary"。列属性可以方便地将某些（标量）SQL表达式映射为沿其他属性加载的“列”。另一方面，您似乎想要映射相关Tag个对象的集合：

class Collection(Base):
    id = Column(Integer, primary_key=True)
    title = Column(Unicode(128), nullable=False)
    posts = relationship("Post", backref="collection", cascade="all,delete-orphan")

    tags = relationship(
        "Tag", viewonly=True,
        primaryjoin="Collection.id == Post.collection_id",
        secondary="join(Post, post_tag)",
        secondaryjoin="Tag.id == post_tag.c.tag_id")

如果你想加载关系，有点像列属性，你可以默认为lazy="join"。还可以使用Query.options()：

在每个查询的基础上定义预先加载策略

session.query(Collection).\
    options(joinedload(Collection.tags)).\
    all()

请注意，您的示例在辅助表post_tags的定义中有拼写错误（？）。列tag_id在名称中包含尾随空格。

如何在SQLAlchemy中聚合连接表中的不同值？

1 个答案: