SQLAlchemy - Using Hybrid Properties in a Query

Question

I am trying to use a SQLAlchemy hybrid property like this

class Metric(Base):
    __tablename__ = 'metric'
    id = Column(Integer, primary_key=True)
    value = Column(Float, nullable=False)

    @hybrid_property
    def dominance(self):
        return 1 - abs(0.5 - float(self.value))

Now I use this in my model as follows

class MetricModel(BaseModel):
    def get_dominance(self):
        self.query(Metric).filter(Metric.dominance >  0.5).order_by(Metric.dominance.desc()).limit(2).all()

This is a flash application and it’s called like that.

model = MetricModel(db)
with db.session():
    print(model.get_dominant_traits())

This gives me an error. TypeError: float() argument must be a string or a number, not 'InstrumentedAttribute' From the error it is clear that there is no result set, therefore, a failure. I followed the docs here http://docs.sqlalchemy.org/en/latest/orm/extensions/hybrid.html What should I do differently?

Answer 1

You need to create expression

from sqlalchemy import func

class Metric(Base):
    __tablename__ = 'metric'
    id = Column(Integer, primary_key=True)
    value = Column(Float, nullable=False)

    @hybrid_property
    def dominance(self):
        return 1 - abs(0.5 - self.value)

    @dominance.expression
    def dominance(cls):
        return 1 - func.abs(0.5 - cls.value)