Django avoids extra queries using prefetch_related not working

Question

I worked previously with select_relatedand prefetch_related, and it worked great.

I am working on the current project, and for some reason I can’t understand why my prefetch query is not working correctly, so I get a lot of redundant database calls.

My models:

class User(models.Model):
    user_extra_info = models.ManyToManyField(
        AppGeneralData,
        through='UserExtraInfo',
        null=True,
        blank=True
    )

class AppGeneralData(models.Model):
    title = models.CharField(max_length=255)
    type = models.PositiveSmallIntegerField(
        choices=GENERAL_DATA_TYPE
    )

class UserExtraInfo(models.Model):
    user = models.ForeignKey(settings.AUTH_USER_MODEL)
    info_item = models.ForeignKey(AppGeneralData)

QuerySet:

User.objects.all().prefetch_related(
    Prefetch(
        'userextrainfo_set',
        queryset=UserExtraInfo.objects.select_related('info_item').all()
    )
)

Problem :

When you repeat the request and call the subquery, it does not extract it from the cached set of requests:

for user in qs:
    user.userextrainfo_set.filter(
        info_item__type=general_data_type
    ).values_list(
        'info_item__title', flat=True
    ))

The subquery just goes to the database on every call, and I don’t understand what is missing.

Thanks.

Answer 1

filter(), , Django prefetch_related.

Prefetch.

qs = User.objects.all().prefetch_related(
    Prefetch('userextrainfo_set', queryset=UserExtraInfo.objects.filter(
        info_item__type=general_data_type
    ).select_related('info_item'), to_attr='general_userextrainfo')
)

filter() .

for user in qs:
    user.general_userextrainfo.values_list('info_item__title', flat=True))