query.filter_by not returning correct type of record - is this a bug?

[scath999]

I have a child model than inherits from a parent.

I want to access a relationship that exists on the child (in this example, "extras"), which is not on the parent.

When I query and find the child, the ORM returns the object as the parent type, making it impossible for me to access the relationship.

This is not happening when I use the terminal.

I'm unsure if this is an issue with my code, flask_sqlalchemy, or sqlalchemy. Guidance is welcome.

Example

class Parent (db.Model):   
    id              = db.Column(db.Integer, primary_key=True, unique=True,)
    first_name      = db.Column(db.String(20))
    last_name       = db.Column(db.String(20))


class Child (Parent):
    __tablename__ = "child"
    __mapper_args__ = {"polymorphic_identity": "child"}

    id          = db.Column(db.Integer, db.ForeignKey("Parent.id"), primary_key=True)
    extras      = db.relationship('Extra', backref='Child', lazy=True)


class Extra(db.Model):
    id           = db.Column(db.Integer, unique=True, primary_key=True)
    child_id     = db.Column(db.Integer, db.ForeignKey("Child.id"), nullable=False)
    text_of_extra = db.Column(db.String, nullable=False)    

A simplified extract of my code looks roughly like:

the_child = Child.query.filter_by(id=child_id).first() # Simplified query demonstrating the point

print(f"child[ {the_child.id} ] has extras: {the_child.extras}")

The result is puzzling:
AttributeError: 'Parent' object has no attribute 'extras'

Note that the object reported is 'Parent' despite the query being made explicitly on 'Child'.

However, from the CLI

>>> with app.app_context():
 ...                 the_child = Child.query.filter_by(id=child_id).first()
 ...
 >>> the_child
<Child 001>               
>>> the_child.extras
<sqlalchemy.orm.dynamic.AppenderQuery object at 0x000002649705F460>