Python Camelot / sqlalchemy：在Select上执行数学运算

Question

I'm using Python Camelot, which uses sqlalchemy and sqlite under the hood. I am storing hours as an integer to avoid floating point errors. For example, 1 hour is stored in the database as 100, 2.5 hours is stored as 250, and so on. When I display hours, I want it to be divided by 100.0. Here's the code:

class Task( Entity ):
    __tablename__ = 'task'
    id = Column( Integer, primary_key=True )
    text = Column( String, nullable=False )
    completed = Column( Boolean, default=False )
    date_created = Column( Date, default=datetime.today )
    date_completed = Column( Date )
    notes = Column( String )

    @property
    def total_hours_estimated (self):
        value = sum([ work.hours_estimated for work in self.work ])
        value = value / 100.0
        return value

    @ColumnProperty
    def total_hours_estimated_colprop (self):
        return sql.select( [sql.func.sum(Work.hours_estimated)],
            Work.task_id == self.id)

class Work( Entity ):
    __tablename__ = 'work'
    id = Column( Integer, primary_key=True )
    date = Column( Date )
    hours_estimated = Column( Integer, default=0 )
    hours_actual = Column( Integer, default=0 )

    task_id = Column( Integer, ForeignKey('task.id') )
    task = relationship(Task, backref='work')

The @property approach calculates the desired result, but when viewed in the Camelot table it doesn't allow you to sort by that column. The docs say you have to use the ColumnProperty approach, but I can't figure out where to divide in the ColumnProperty approach. Is it possible?

Edit : It seems the issue is if I add in a float division, the sql.select is giving back Decimal objects, and maybe Camelot isn't liking that. Example:

>>> session.query(sql.select( [sql.func.sum(Work.hours_estimated/100.0)])).all()
[(Decimal('5.5000000000'),)]

Answer 1

This is because the SQLAlchemy introspection is not able to exactly know the return type when some math is involved, in this case, it's probably the precision of the result that is unknown, so you have to help it a bit. This can be don through the field attributes :

class Task(Entity):
    ...
    class Admin(EntityAdmin):
        list_display = ['id', 'text', 'total_hours_estimated_colprop']
        field_attributes = {
            'total_hours_estimated_colprop': {'delegate':delegates.FloatDelegate',
                                              'precision': 3} }

This forces displaying the result as float with precision 3.