获取子列的平均值，但来自 SQLAlchemy 中过滤的父数据

Question

If I have a one-to-many relationship (where a parent can have many children) like so:

class Parent(Base):
    __tablename__ = 'parent'
    id = Column(Integer, primary_key=True)
    score = Column(Integer, nullable=False)
    children = relationship("Child", back_populates='parent')

class Child(Base):
    __tablename__ = 'child'
    id = Column(Integer, primary_key=True)
    data = Column(Integer, nullable=False)
    parent = relationship("Parent", back_populates='children')

I want all children whose parents have 70 < score < 100 , and of those children, I want the average of all their data .

I know I can get all the children with:

childs = session.query(Parent).filter(Parent.score > 70, Parent.score < 100)

for c in childs:
    # calculate the average 'data' here...

But this is probably not efficient and is very manual way of calculating the average child data.

I know there is this way of getting an average of a table:

 session.query(func.avg(Child.data))

But how can I tie this into the above query where I filter on the parent's score?

Answer 1

我认为您可以应用与其他查询相同的过滤器：

 session.query(func.avg(Child.data)).filter(Child.parent.score > 70, Child.parent.score < 100)