Question: How to override queryset count() method in Django's admin list

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How to override queryset count() method in Django's admin list

Answers 2
Added at 2017-01-04 16:01
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In order to avoid time consuming and costly exact database count queries, I'd like to override the count() method inside a Django admin class like so:

from django.contrib import admin
from django.db import connection

class CountProxy:
    def __call__(self):
        # how to access the queryset `query` here?
        query = ...

        try:
            if not query.where:
                cursor = connection.cursor()
                cursor.execute("SELECT reltuples FROM pg_class WHERE relname = %s", [query.model._meta.db_table])
                n = int(cursor.fetchone()[0])
                if n >= 1000: return n # exact count for small tables
            return object_list.count()
        except:
            # exception for lists
            return len(object_list)
        return estimated_count

class MyAdmin(admin.ModelAdmin):
    def get_queryset(self, request):
        qs = super(MyAdmin, self).get_queryset(request)
        qs.count = CountProxy()
        return qs

But I don#t know how to access the original queryset within my CountProxy class. Any idea? I know I can overwrite the whole changelist view through get_changelist. But that involves a lot of duplicating code from Django's repo.

Answers to

How to override queryset count() method in Django's admin list

nr: #1 dodano: 2017-01-04 16:01

I did something similar before so I can help.

I defined a custom queryset class:

class MyQuerySet(QuerySet):

    def count(self):
        """
        Override count queries (performed by Django ORM) to display approximate value.
        This will speed the admin interface.

        """
        if self._result_cache is not None and not self._iter:
            return len(self._result_cache)

        query = self.query
        if not (query.group_by or query.having or query.distinct):
            cursor = connections[self.db].cursor()
            cursor.execute("SHOW TABLE STATUS LIKE '%s';" % self.model._meta.db_table)
            return cursor.fetchall()[0][4]
        else:
            return self.query.get_count(using=self.db)

Then defined a custom model manager:

class MyManager(models.Manager):

    def get_query_set(self):
        return MyQuerySet(self.model)

Then used it in my model:

class MyModel(models.Model):
    objects = MyManager()
nr: #2 dodano: 2017-01-04 16:01

I could be wrong, but could you pass qs as an instance attribute for CountProxy?

class CountProxy:
    def __init__(self, query):
        self.query = query

    def __call__(self):
        # you've already had the query here, do something with self.query

class MyAdmin(admin.ModelAdmin):
    def get_queryset(self, request):
        qs = super(MyAdmin, self).get_queryset(request)
        qs.count = CountProxy(qs)
        return qs
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