date and integer addition in django F object

I am trying retrieve objects with the following model

Tools:
    last_calibration_date: Date
    period:Integer
    .
    .

Now to get all expired tools, I want to perform a query which does something like last_calibration_date + period < current_date

How can t开发者_高级运维his be done using django ORM? Its easy with raw query. I am trying something like

.filter(last_calibration_date__gte = date.today()+ timedelta(F('calibration_period')*30))

Not working! Is it possible?

No, F objects don't work that way. You need to use extra() - for instance, using MySQL:

.extra(where=['period + last_calibration_date < CURDATE()']