CODEWITHSUNDEEP
sqldjangodjango-querysetextra
jnns
jnns

Reputation: 5634

Using extra() on ValuesQuerySet in Django

I'm trying to calculate a percentage with two values which are themselves aggregated. The SQL query that explains what I'm after is as follows:

SELECT (SUM(field_a) / SUM(field_b) * 100) AS percent
FROM myapp_mymodel 
GROUP BY id
ORDER BY id

I tried to use the following to construct a QuerySet, but unfortunately it doesn't contain the extra field:

MyModel.objects.values('id').annotate(
   sum_field_a=Sum('field_a'),
   sum_field_b=Sum('field_b')).extra(
      select={'percent': 'sum_field_a / sum_field_b * 100'})

What irritates me is that - according to the Django documentation - this seems to be the way to go:

When a values() clause is used to constrain the columns that are returned in the result set […] instead of returning an annotated result for each result in the original QuerySet, the original results are grouped according to the unique combinations of the fields specified in the values() clause. An annotation is then provided for each unique group; the annotation is computed over all members of the group.

Source: http://docs.djangoproject.com/en/dev/topics/db/aggregation/#values

If you use a values() clause after an extra() clause, any fields defined by a select argument in the extra() must be explicitly included in the values() clause. However, if the extra() clause is used after the values(), the fields added by the select will be included automatically.

Source: http://docs.djangoproject.com/en/dev/ref/models/querysets/#values

Upvotes: 9

Views: 19312

Answers (3)

hynekcer
hynekcer

Reputation: 15548

Aggregate expressions allow such expressions on Aggregate functions easily since Django 1.8 without the problematic 'extra()' method.

qs = (
    MyModel.objects.values('id')
    .annotate(percent=Sum('field__a') / Sum('field__b') * 100)
    .order_by('id')
)

>>> print(str(qs.query))
SELECT id, ((SUM(field_a) / SUM(field_b)) * 100) AS percent
FROM app_mymodel GROUP BY id ORDER BY id ASC

(The mentioned issue #15546 has been closed soon by a documentation note that extra() after values() will not work - commit a4a250a.)

Upvotes: 4

Thomas
Thomas

Reputation: 11888

If you use a values() clause after an extra() clause, any fields defined by a select argument in the extra() must be explicitly included in the values() clause.

Source: http://docs.djangoproject.com/en/dev/ref/models/querysets/#values

the 'percent' field added in the select can be explicitly added to the values clause and it should be added to the queryset.

MyModel.objects.annotate(
              sum_field_a=Sum('field_a'),
              sum_field_b=Sum('field_b')).extra(
              select={'percent': 'sum_field_a / sum_field_b * 100'}
         ).values('id', 'percent')

Upvotes: 1

Xixi
Xixi

Reputation: 329

As you pointed out (#15546) there may be a bug in django.

But as a workaround, you can place the burden of the actual computation on python instead of the SQL database, by doing something like this:

[{'field_c': model['field_c'],
  'percent': m['sum_field_a'] * 100.0 / m['sum_field_b']}
 for model in MyModel.objects.values('field_c').annotate(
    sum_field_a=Sum('field_a'),
    sum_field_b=Sum('field_b')).order_by('field_c')]

As this solution forces you to loop through all the data, depending on what you want to do it may or may not be acceptable.

Upvotes: 0

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