Django 加权百分比

Question

I'm trying to calculate a weighted percentage in a Django query.

This is an example of what my data looks like:

id      start_date  agency_id   area_id housetype_id    no_of_changed   price_change_percentage total
6716    2017-08-26  11          1       1               16              -0.09                   35
6717    2017-08-26  11          1       3               44              -0.11                   73
6718    2017-08-26  11          1       4               7               -0.1                    12
6719    2017-08-26  11          1       5               0               0                       4
6720    2017-08-26  11          1       6               0               0                       1
6721    2017-08-26  21          1       1               0               0                       1
6722    2017-08-26  34          1       1               0               0                       1
6723    2017-08-26  35          1       1               0               0                       1
6724    2017-08-26  38          1       1               0               0                       1

and this is my current code:

from django.db.models import F, FloatField, ExpressionWrapper
from app.models import PriceChange

def weighted_percentage(area_id, date_range, agency_id, housetype):

    data = PriceChange.objects.filter(area_id=area_id,
                                      start_date__range=date_range,
                                      agency_id=agency_id,
                                      )

    if housetype:
        data = data.filter(housetype=housetype) \
            .values('start_date') \
            .annotate(price_change_total=ExpressionWrapper((F('price_change_percentage') * F('no_of_changed')) / F('total'), output_field=FloatField())) \
            .order_by('start_date')

    else:
        # what to do?
        pass

    x = [x['start_date'] for x in data]
    y = [y['price_change_total'] for y in data]

    return x, y

I figured out how to do the calculation when housetype is defined and I only need to data from one row. I can't figure out how to done when I need to calculate for multiple rows with the same start_date. I don't want a value for each row but for each start_date.

As an example (two rows with same start_date, area_id, agency_id but different housetype_ids): no_of_changed price_change_percentage total 16 -0.09 35 44 -0.11 73

The calculation is in pseudocode: ((no_of_changed[0] * price_changed_percentage[0]) + (no_of_changed[0] * price_changed_percentage[0])) / (total[0] + total[1]) = price_change_total

((16 * -0.09) + (44 * -0.11)/ (35 + 73) = -0.03148148

I'm using Django 1.11 and Python 3.6.

Answer 1

You need to wrap the expression in a Sum expression.

Add the following import:

from django.db.models import Sum

Then add the following query

else:
    data = data.values('start_date') \
       .annotate(
           price_change_total=ExpressionWrapper(
               Sum(F('price_change_percentage') * F('no_of_changed')) / Sum(F('total')),
               output_field=FloatField()
           )
       ) \
       .order_by('start_date')

What is happening here is that when you use an aggregation expression such as Sum inside an annotate() call, it is translated into a group_by query in the database. All columns listed in the previous values() clause are used to create the group_by query.

See this blog post for further explanation and a breakdown of the resulting SQL query.