如何按关系 model 中的列的 hybrid_property 排序？

Question

I need to order some Parent objects when i do query by child_field.
I try to do it like this:

stmt = select(Parent).where([<conditions>]).options([joinedload(Parent.child)]).order_by(desc(Parent.child_field)))
# I have more options in real code

while my models looks like these:

class Parent(Base):
    __table_name___ = "parent"
    some_field = Column(String(256))
    child = relationship("Child", back_populates="parent")])

    @hybrid_property
    def child_field(self):
        return self.child.child_field

class Child(base):
    child_field = Column(Integer)
    parent = relationship("Parent", back_populates="child")

I get error like this which discussed here :

AttributeError: Neither 'InstrumentedAttribute' object nor 'Comparator' object associated with Parent.child has an attribute 'child_field'

However solution I can't properly understand or it's not apply in my case.

So by reading docs i concluded that I need to write custom comparator:

class CustomComparator(Comparator):
    def operate(self, op, other, **kwargs):
        return op(self.__clause_element__(), other, **kwargs)

and added like this to parent model

# --- snip ---
    @child_field.comparator
    def child_field(cls):
        return CustomComparator(cls.child_field)

But this implementation gives me recursion error of course.

Can anyone explain what's wrong and how to achieve my goal because apparently docs nor Comparator class insides does explain it me?
+
Should I even solve it with custom comparator when hybrid_property returns just an int ?

UPD: I tried use expression instead of comparator, and stopped getting comparator object related error, however query return not ordered list.

# ---snip---
@child_field.expression
def child_filed(cls):
    return Child.child_field

Answer 1

Should be working with an expression. Try to change your expression to the following:

@child_field.expression
def child_field(cls):
    return cls.child.child_field