validate column of polymorphic table - sqlalchemy

Question

I have a flask-sqlalchemy polymorphic table structure like so
class Parent(db.Model):
id = db.Column(db.Integer, primary_key=True)
polytype = db.Column(db.String(32), nullable=False)
value = db.Column(db.String(32))
__mapper_args__ = {'polymorphic_identity': 'parent',
'polymorphic_on': polytype}
class Child(Parent):
id = db.Column(db.Integer,db.ForeignKey('parent.id'),
primary_key=True)
#validates('value')
def validate_value(self, key, val):
# [validation code]
return value
__mapper_args__ = {'polymorphic_identity': 'child'}
and I want to validate the value field. However, the validator for Child.value, the column inherited from Parent, never runs.
What is the correct way to validate an inherited column?

Answer 1

There's an old open issue about it.
In that issue, it is suggested that using an event listener can work, for example:
from flask_sqlalchemy import SQLAlchemy
from sqlalchemy import event
db = SQLAlchemy()
class Parent(db.Model):
id = db.Column(db.Integer, primary_key=True)
polytype = db.Column(db.String(32), nullable=False)
value = db.Column(db.String(32))
__mapper_args__ = {'polymorphic_identity': 'parent',
'polymorphic_on': polytype}
class Child(Parent):
id = db.Column(db.Integer,db.ForeignKey('parent.id'),
primary_key=True)
__mapper_args__ = {'polymorphic_identity': 'child'}
#event.listens_for(Parent.value, "set", propagate=True)
def validate_value(inst, val, *args):
print(f"checking value for {inst}")
assert val == "spam"
Parent(value="spam")
Child(value="spam")
If you don't want the listener to fire on Parent instances, decorate your listener func with event.listens_for(Child.value, ...).

Answer 2

Another workaround for this known open issue is creating an instance method to be called inside the #validate method, for example:
from flask_sqlalchemy import SQLAlchemy
from sqlalchemy.orm import validates
from sqlalchemy import event
db = SQLAlchemy()
class Parent(db.Model):
id = db.Column(db.Integer, primary_key=True)
polytype = db.Column(db.String(32), nullable=False)
value = db.Column(db.String(32))
__mapper_args__ = {'polymorphic_identity': 'parent',
'polymorphic_on': polytype}
#validates('value')
def validate_value(self, key, new_value):
self._validate_value(new_value)
def _validate_value(self, new_value):
print(f"checking value for {self}")
assert new_value == "spam"
class Child(Parent):
id = db.Column(db.Integer,db.ForeignKey('parent.id'),
primary_key=True)
__mapper_args__ = {'polymorphic_identity': 'child'}
def _validate_value(self, new_value):
print(f"checking value for {self}")
assert new_value == "child"
Parent(value="spam")
Child(value="child")
In this case, you are able to validate the value having different behaviors on the Childs overwriting the private method _validate_value