如何在 SQLAlchemy 的连接表中聚合不同的值？

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。

我想 Collection.tags 返回一组不同的帖子标签集。

但是，当我访问Collection.tags 时出现以下错误：

sqlalchemy.exc.OperationalError: (sqlite3.OperationalError) 对于作为表达式一部分的 SELECT 只允许一个结果

编辑

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")) 是错误的，因为 post_id 在 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

我改成

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 SQLAlchemy 生成的查询没有

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

您需要relationship() with a composite "secondary" 而不是column_property()。列属性对于将某些（标量）SQL 表达式映射为与其他属性一起加载的“列”很方便。另一方面，您似乎想要映射一组相关的 Tag 对象：

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")

如果你想预先加载关系，有点像列属性，你可以默认为lazy="join"。也可以使用Query.options() 为每个查询定义预先加载策略：

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

请注意，您的示例在辅助表post_tags 的定义中有错字（？）。 tag_id 列的名称中有尾随空格。