Class: ColumnProperty

A specialized form of the GenericProperty to generate SQLAlchemy column_property's.

It takes a function (often given as an anonymous lambda) as its first argument. Other arguments and keyword arguments are forwarded to the column_property construct. That first-argument function must accept exactly one argument and must return the desired (scalar-returning) SQLAlchemy ClauseElement.

The function will be called whenever the entity table is completely defined, and will be given the .c attribute of the table of the entity as argument (as a way to access the entity columns). The ColumnProperty will first wrap your ClauseElement in an "empty" label (ie it will be labelled automatically during queries), then wrap that in a column_property.

class OrderLine(Entity):
    quantity = Field(Float)
    unit_price = Field(Numeric)
    price = ColumnProperty(lambda c: c.quantity * c.unit_price,
                           deferred=True)