[Django]-Using .aggregate() on a value introduced using .extra(select={…}) in a Django Query?

by

15👍

✅

You could use a custom aggregate function to produce your query:

WEEK_FUNC = 'STRFTIME("%%%%W", %s)' # use 'WEEK(%s)' for mysql

class WeekCountAggregate(models.sql.aggregates.Aggregate):
    is_ordinal = True
    sql_function = 'WEEK' # unused
    sql_template = "COUNT(%s)" % (WEEK_FUNC.replace('%%', '%%%%') % '%(field)s')

class WeekCount(models.aggregates.Aggregate):
    name = 'Week'
    def add_to_query(self, query, alias, col, source, is_summary):
        query.aggregates[alias] = WeekCountAggregate(col, source=source, 
            is_summary=is_summary, **self.extra)


>>> game_objects.extra(select={'week': WEEK_FUNC % '"games_game"."date"'}).values('week').annotate(count=WeekCount('pk'))

But as this API is undocumented and already requires bits of raw SQL, you might be better off using a raw query.

👤emulbreh

4👍

Here is an example of the problem and an unideal workaround solution. Take this example model:

class Rating(models.Model):
    RATING_CHOICES = (
        (1, '1'),
        (2, '2'),
        (3, '3'),
        (4, '4'),
        (5, '5'),
    )
    rating = models.PositiveIntegerField(choices=RATING_CHOICES)
    rater = models.ForeignKey('User', related_name='ratings_given')
    ratee = models.ForeignKey('User', related_name='ratings_received')

This example aggregate query fails in the same way as yours because it attempts to reference a non-field value created using .extra().

User.ratings_received.extra(
    select={'percent_positive': 'ratings > 3'}
).aggregate(count=Avg('positive'))

One Workaround Solution

The desired value can be found directly by using the aggregate database function (Avg in this case) within the extra value’s definition:

User.ratings.extra(
    select={'percent_positive': 'AVG(rating >= 3)'}
)

This query will generate the following SQL query:

SELECT (AVG(rating >= 3)) AS `percent_positive`,
       `ratings_rating`.`id`,
       `ratings_rating`.`rating`,
       `ratings_rating`.`rater_id`,
       `ratings_rating`.`ratee_id`
FROM `ratings_rating`
WHERE `ratings_rating`.`ratee_id` = 1

Despite the unneeded columns in this query, we can still obtain the desired value from it by isolating the percent_positive value:

User.ratings.extra(
    select={'percent_positive': 'AVG(rating >= 3)'}
).values('percent_positive')[0]['percent_positive']
👤Trey Hunner

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