Unable to add rows to an inherited table with SQLAlchemy v1.4. I get a "NULL result in a non-nullable column" error

For the sake of this example I have four tables:

1. ModelType
2. ModelTypeA
3. ModelTypeB
4. Model

I am trying to model the following relationship among these tables:

To do that I am defining the classes using SQL Alchemy v1.4 in the following way:

Base = declarative_base()

class ModelType(Base):
    __tablename__ = "modeltype"

    id = Column(Integer, primary_key=True)
    algorithm = Column(String)

    models = relationship("Model", back_populates="modeltype")

    __mapper_args__ = {
        "polymorphic_identity": "modeltype",
        "polymorphic_on": algorithm
    }

    def __repr__(self):
        return f"{self.__class__.__name__}({self.algorithm!r})"

class ModelTypeA(ModelType):
    __tablename__ = "modeltypea"

    id = Column(Integer, ForeignKey("modeltype.id"), primary_key=True)
    parameter_a = Column(Integer)

    __mapper_args__ = {
        "polymorphic_identity": "Model Type A"
    }
    
class ModelTypeB(ModelType):
    __tablename__ = "modeltypeb"

    id = Column(Integer, ForeignKey("modeltype.id"), primary_key=True)
    parameter_a = Column(Integer)

    __mapper_args__ = {
        "polymorphic_identity": "Model Type B"
    }

class Model(Base):
    __tablename__ = "model"

    id = Column(Integer, primary_key=True)
    trainingtime = Column(Integer)
    modelversionid = Column(Integer, ForeignKey("modeltype.id"))
    
    modeltype = relationship("ModelType", back_populates="models")

    def __repr__(self) -> str:
        return f"Model(id={self.id!r}, trainingtime={self.trainingtime!r})"

I am able to create the tables with:

with Session(engine) as session:
    Base.metadata.create_all(engine)

This emits the following create table sql statements:

CREATE TABLE modeltype (
        id INTEGER NOT NULL AUTOINCREMENT, 
        algorithm VARCHAR, 
        PRIMARY KEY (id)
)

CREATE TABLE modeltypea (
        id INTEGER NOT NULL, 
        parameter_a INTEGER, 
        PRIMARY KEY (id), 
        FOREIGN KEY(id) REFERENCES modeltype (id)
)

CREATE TABLE modeltypeb (
        id INTEGER NOT NULL, 
        parameter_b INTEGER, 
        PRIMARY KEY (id), 
        FOREIGN KEY(id) REFERENCES modeltype (id)
)

CREATE TABLE model (
        id INTEGER NOT NULL AUTOINCREMENT, 
        trainingtime INTEGER, 
        modelversionid INTEGER, 
        PRIMARY KEY (id), 
        FOREIGN KEY(modelversionid) REFERENCES modeltype (id)
)

This looks good to me. But now when I try to actually create a model with reference to a modeltype like so:

model_type_a = ModelTypeA(parameter_a=3)

model = Model(trainingtime=10, modeltype=model_type_a)

session.add(model)
session.commit()

I get the following warning:

SAWarning: Column 'modeltypea.id' is marked as a member of the primary key for table 'modeltypea', but has no Python-side or server-side default generator indicated, nor does it indicate 'autoincrement=True' or 'nullable=True', and no explicit value is passed. Primary key columns typically may not store NULL.

and then the following error:

sqlalchemy.exc.IntegrityError: (snowflake.connector.errors.IntegrityError) 100072 (22000): NULL result in a non-nullable column [SQL: INSERT INTO modeltypea (parameter_a) VALUES (%(parameter_a)s)] [parameters: {'parameter_a': 3}] (Background on this error at: https://sqlalche.me/e/14/gkpj)

So it seems like when I create an instance of the inherited class ModelTypeA, it is not creating a row in ModelType and thus it has no id in ModelType to refer to and is instead trying to use NULL. How am I supposed to add rows when I have this sort of inheritance?