Quick example of using class-decorators to declare compound columns

sqlalchemy-scalar.py

from sqlalchemy import create_engine
from sqlalchemy import Column, Float, Integer, String
from sqlalchemy.ext.declarative import declarative_base

engine = create_engine('sqlite:///:memory:', echo=True)
Base = declarative_base()


class ScalarField:
    pass


def make_scalars(class_):
    for attr_name in dir(class_):
        attr_value = getattr(class_, attr_name)
        if isinstance(attr_value, ScalarField):
            setattr(class_, f'{attr_name}_unit', Column(String))
            setattr(class_, f'{attr_name}_value', Column(Float))
    return class_


@make_scalars
class Fluid(Base):
    __tablename__ = 'fluid'

    id = Column(Integer, primary_key=True)
    density = ScalarField()

    def __repr__(self):
       return f"<Fluid(density_value={self.density_value}, density_unit='{self.density_unit}')>"


Base.metadata.create_all(engine)

f = Fluid(density_value=1000.0, density_unit='kg/m3')

from sqlalchemy.orm import sessionmaker
Session = sessionmaker(bind=engine)

session = Session()
session.add(f)

print(session.query(Fluid).first())

2019-03-26 13:09:40,806 INFO sqlalchemy.engine.base.Engine SELECT CAST('test plain returns' AS VARCHAR(60)) AS anon_1
2019-03-26 13:09:40,806 INFO sqlalchemy.engine.base.Engine ()
2019-03-26 13:09:40,806 INFO sqlalchemy.engine.base.Engine SELECT CAST('test unicode returns' AS VARCHAR(60)) AS anon_1
2019-03-26 13:09:40,806 INFO sqlalchemy.engine.base.Engine ()
2019-03-26 13:09:40,807 INFO sqlalchemy.engine.base.Engine PRAGMA table_info("fluid")
2019-03-26 13:09:40,807 INFO sqlalchemy.engine.base.Engine ()
2019-03-26 13:09:40,808 INFO sqlalchemy.engine.base.Engine 
CREATE TABLE fluid (
	id INTEGER NOT NULL, 
	density_unit VARCHAR, 
	density_value FLOAT, 
	PRIMARY KEY (id)
)


2019-03-26 13:09:40,808 INFO sqlalchemy.engine.base.Engine ()
2019-03-26 13:09:40,808 INFO sqlalchemy.engine.base.Engine COMMIT
2019-03-26 13:09:40,809 INFO sqlalchemy.engine.base.Engine BEGIN (implicit)
2019-03-26 13:09:40,810 INFO sqlalchemy.engine.base.Engine INSERT INTO fluid (density_unit, density_value) VALUES (?, ?)
2019-03-26 13:09:40,810 INFO sqlalchemy.engine.base.Engine ('kg/m3', 1000.0)
2019-03-26 13:09:40,811 INFO sqlalchemy.engine.base.Engine SELECT fluid.id AS fluid_id, fluid.density_unit AS fluid_density_unit, fluid.density_value AS fluid_density_value 
FROM fluid
 LIMIT ? OFFSET ?
2019-03-26 13:09:40,811 INFO sqlalchemy.engine.base.Engine (1, 0)
<Fluid(density_value=1000.0, density_unit='kg/m3')>