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 like this

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 flask app and it's being called like this

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 looks like there is no result set, hence the failure. I followed the docs here http://docs.sqlalchemy.org/en/latest/orm/extensions/hybrid.html What should I do differently?

r-m-n · Accepted Answer · 2017-02-27 19:31:16Z

28

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)

answered Feb 27, 2017 at 19:31

r-m-n

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2 Comments

user2682863 Over a year ago

This link provides a pretty good explanation docs.sqlalchemy.org/en/13/orm/mapped_sql_expr.html

MattSom Over a year ago

But what if the data is on the instance level and different for every object (using another hybrid property in the expression)?

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