SQLAlchemy Table inheritance with composite key?

Question

I am trying to create Class Table Inheritance with a composite key on the base table. I am trying to create sometime similar to this answer. However, the tables I am creating are EntityWithImages, Experiment, Sample, and Images; my code so far is,

class EntityWithImages(db.Model):
    __tablename__ = "entity_images"

    id = db.Column(db.Integer, primary_key=True)
    entity_type = db.Column(db.String(32), nullable=False)

    __mapper_args__ = {
        "polymorphic_on": entity_type,
        "polymorphic_identity": "entity_with_images",
    }


class Experiment(TimeStampAuditMixin, EntityWithImages):
    __tablename__ = "experiment"

    # id = db.Column(db.Integer, primary_key=True)
    id = db.Column(None, db.ForeignKey('entity_images.id'), primary_key=True)
    experiment_name = db.Column(db.Text, unique=True)

    samples = db.relationship("Sample", back_populates="experiment", foreign_keys="Sample.id")

    __mapper_args__ = {
        "polymorphic_identity": "experiment",
    }

    def __str__(self):
        return self.experiment_name


class Sample(TimeStampAuditMixin, EntityWithImages):
    __tablename__ = "sample"

    # id = db.Column(db.Integer, primary_key=True)
    id = db.Column(None, db.ForeignKey('entity_images.id'), primary_key=True)
    sample_name = db.Column(db.Text, unique=True)
    experiment_id = db.Column(db.ForeignKey("experiment.id", ondelete="CASCADE"))
    
    experiment = db.relationship("Experiment", back_populates="samples", foreign_keys=experiment_id)

    id: int
    sample_name: str
    s3_uri: str
    experiment_id: int
    experiment: "Experiment"

    __mapper_args__ = {
        "polymorphic_identity": "sample",
    }

    def __str__(self):
        return self.sample_name


class ImageSource(db.Model):
    __tablename__ = "image_source"

    id = db.Column(db.Integer, primary_key=True)
    entity_images_id = db.Column(db.ForeignKey("entity_images.id", ondelete="CASCADE"))

I am not clear on how to detail the value for the entity_type in the corresponding Experiment and Sample tables, i.e. in the Experiment table I want the default entity_type to be experiment.

I am currently using SQLAlchemy==1.4.46