Polymorphic + Hybrid

measurement.py

from sqlalchemy import Column, Integer, Text, Float
from sqlalchemy import func
from sqlalchemy.dialects.postgresql import JSONB
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy.ext.hybrid import hybrid_property
from sqlalchemy.orm import Session


Base = declarative_base()


class Measurement(Base):

    __tablename__ = 'measurements'

    type = Column(Text, nullable=True)

    __mapper_args__ = {'polymorphic_on': type}

    id = Column(Integer, primary_key=True)

    value = Column(Integer, nullable=False)

    logger = Column(Integer, nullable=True)

    channel = Column(Integer, primary_key=True)

    meta = Column(JSONB, default=dict)


class MeasurementA(Measurement):

    __mapper_args__ = {'polymorphic_identity': 'a'}

    @hybrid_property
    def corrected_value(self):
        return (self.value >> 4) * 0.9987

    @corrected_value.expression
    def corrected_value(cls):
        return func.cast(cls.value.op('>>')(4), Float) * 0.9987


print(
    Session().query(MeasurementA.corrected_value).filter(
        MeasurementA.logger == 1,
        MeasurementA.channel == 2
    )
)

# SELECT CAST(measurements.value >> :value_1 AS FLOAT) * :param_1 AS corrected_value
# FROM measurements
# WHERE measurements.logger = :logger_1 AND measurements.channel = :channel_1 AND measurements.type IN (:type_1)