Using model methods in aggregates

I am trying to use a model method in a django aggregate query. I'm not sure if it is possible and I may be going at this the wrong way.

Here is the model I wish to query.

class ClassNumbers(models.Model):
   """
   The class year level and number inline model for a booking
   """
   booking = models.Forei开发者_运维百科gnKey('Booking')
   yearLevel = models.CharField(max_length=10, choices=YEAR_CHOICES, verbose_name='Year Level')
   numberOfStudents = models.IntegerField(verbose_name='Number of Students') 

class Booking(models.Model):
   # A shorter version of the model
   date = models.DateField()
   institution = models.ForeignKey(Institution)

   def getStudentTotal(self):
      # Total class numbers
      classes = ClassNumbers.objects.filter(booking=self.id)
      classTotal = 0
      if ( classes ):
          for c in classes:
              classTotal += c.numberOfStudents
      return classTotal

   def getHDSV(self):
      HDSVunits = {
                'Full-Day': 2.0,
                'Half-Day AM': 1.0,
                'Half-Day PM': 1.0,
                'Three-Quarter Day': 1.5,
                '1 Hour': 0.5,
                'Custom': 1.0,
                }
      numStudents = self.getStudentTotal()
      result = numStudents * HDSVunits[self.price.name]
      return result

The getHDSV method returns a reporting metric used internally where the app lives. I wish to aggregate the metric into a total for the month between a date period.

I am no aggregate/annotate master. My attempts so far have not netted the results I'm after.

Ultimately I queried Bookings between the specified dates and then looped over the results and tallied the reporting unit into a dictionary by calling the getHDSV method each iteration. Of course the resulting dictionary is not sorted the way I would like. So I am now turning to get some help.

Given the way the metric is generated can I call a model method while aggregating data in a query? Or should I be using the HDSVunits dictionary while creating the aggregate? Or is there a better way?

Thanks.

You've got quite a difficult setup, it might be easier to have the HDSVunits mapping on the Price model to make it easier to access in queries.

The best I can come up with is something like this:

Booking.objects.aggregate(
    hdsv=(
        Sum('classnumbers__numberofstudents') * 
        Case(
            When(price__name='Full-Day', then=2.0),
            When(price__name='Half-Day AM', then=1.0),
            When(price__name='Full-Day PM', then=1.0),
            When(price__name='Three-Quarter Day', then=1.5),
            When(price__name='1 Hour', then=0.5),
            When(price__name='Custom', then=1.0),
            output_field=FloatField(),
        )
    )
)

If the HDSV value were stored as a field on the Price model, you could simply do:

Booking.objects.aggregate(
    hdsv=Sum('classnumbers__numberofstudents') * F('price__hdsv'))

On a side note, you should really consider following the Python naming convensions which would make it easier for other Python developers to help you.

If the data that getHDSV is returning is not from the database then aggregate and annotate will not be able to be used to gather statistics on it.