Question

如果我指定了joinedload，SQLAlchemy可以急切地加载集合的内容选项。但是，我有一个案例，我实际上并不感兴趣的是集合的内容，只是其中的元素数量。

是否可以让SQLAlchemy作为查询的一部分急切地获取集合的大小？

例如，假设我有一个类似的结构（真实的例子是冗长的）

class Person:
  name = Column(String)
  avatarUrl = Column(String)
  comments = relation(Comment)

class Wall:
  Person for_whom

class Comment
  commenter = relation(Person)
  wall = relation(Wall)
  text = Column(String)

现在（再次抽象地说）如果我在墙上收到评论列表，我还可以获得评论者发布的评论总数吗？

session.query(Comment)
    .filter(Comment.wall == wall)
    .options(joinedload("commenter"))
    .options(joinedcount("commenter.comments")) # Here's the mysterious part
    .all()

Answer 1

# alias comments table because it will appear twice in query
comments = aliased(Comment)
result = (session.query(Comment, func.count(comments.id))
    .filter(Comment.wall==wall)
    .join(Person) # we need to join person table explicitly
    .join(comments) # to reach comments table again in another join 
    .group_by(Comment.id)
     # populates relationship using explicitly joined table
    .options(contains_eager(Comment.commenter))
    .all())

SQLAlchemy渴望加载集合大小/计数

1 个答案: