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class Training(db.Model):
    __tablename__ = 'trainings'
    id = db.Column(db.Integer, primary_key=True, index=True)
    (...)
    #relationships
    slot_id = db.Column(db.Integer, db.ForeignKey('slots.id')) #foreign key to the table name


class Slot(db.Model):
    __tablename__ = 'slots'
    id = db.Column(db.Integer, primary_key=True, index=True)
    (...)
    taken = db.Column(db.Boolean(), index=True)
    #relationships
    trainings = db.relationship('Training', backref='slots', uselist=False)


example = Training.query.filter_by(Training.slots.taken==True).first()

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AttributeError: Neither 'InstrumentedAttribute' object nor 'Comparator' object associated with Training.slots has an attribute 'taken'

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example = Training.query.join(Slot).filter_by(taken=True).first()

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sqlalchemy.exc.InvalidRequestError: Entity namespace for "trainings" has no property "taken"

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example = Training.query.join(Slot).filter(Slot.taken==True).first()

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SAWarning: SELECT statement has a cartesian product between FROM element(s) "slots" and FROM element "trainings".  Apply join condition(s) between each element to resolve.

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class Slot(db.Model):
    __tablename__ = 'slots'
    id = db.Column(db.Integer, primary_key=True, index=True)
    (...)
    taken = db.Column(db.Boolean(), index=True)
    #relationships
    trainings = db.relationship('Training', backref='plaetze', uselist=False)