How to load SQL expression as mapped attribute having relationship with SQLAlchemy?

Let's have two SQLAlchemy models:

class Child(Base):
    __tablename__ = "child"
    id = Column(String, primary_key=True)
    parent_id = Column(String, ForeignKey("parent.id"), nullable=False)

class Parent(Base):
    __tablename__ = "parent"
    id = Column(String, primary_key=True)
    children = relationship("Child", lazy="joined")

What I try to achieve is having a boolean attribute (let's call it has_deps) on Parent which will be True when there are children for a given parent.

I have also some constraints:

• It must be accessible when I convert the object to a dict
• It must not generate a correlated subquery

I tried different methods :

• Using column_property with exists gave me a correlated subquery

Parent.has_deps = column_property(
    exists([Child.parent_id]).where(Child.parent_id == Parent.id)
)

results in :

SELECT parent.id AS parent_id, EXISTS (SELECT child.parent_id FROM child WHERE child.parent_id = parent.id) AS anon_1, parent.id AS id_1, child_1.id AS child_1_id, child_1.parent_id AS child_1_parent_id 
FROM parent LEFT OUTER JOIN child AS child_1 ON parent.id = child_1.parent_id

• Using an hybrid_property is not loaded at query time nor accessible with dict and has no purpose for that

@has_deps.expression
def has_deps(cls):
    return case([(Child.parent_id != None, True)], else_=False)

(will not even be linked to the left outer alias join generated by the relationship)

As an example, this is the query I want the ORM to produce by just doing session.query(Parent).all():

SELECT parent.id AS parent_id, CASE WHEN child_1.parent_id IS NOT NULL THEN true ELSE false END AS has_deps, parent.id AS id_1, child_1.id AS child_1_id, child_1.parent_id AS child_1_parent_id 
FROM parent LEFT OUTER JOIN child AS child_1 ON parent.id = child_1.parent_id