from django.contrib.contenttypes.models import ContentType
from django.contrib.comments.models import Comment
from django.db import connection

qn = connection.ops.quote_name

def qf(table, field): # quote table and field
    return '%s.%s' % ( qn(table), qn(field) )

def comments_extra_count(queryset):
    
    commented_model = queryset.model
    contenttype = ContentType.objects.get_for_model(commented_model)
    commented_table = commented_model._meta.db_table
    comment_table = Comment._meta.db_table
    
    sql = '''SELECT COUNT(*) FROM %s
        WHERE %s=%%s AND %s=%s
    ''' % (
        qn(comment_table),
        qf(comment_table, 'content_type_id'),
        qf(comment_table, 'object_pk'),
        qf(commented_table, 'id')
    )
    
    return queryset.extra(
        select={'comment_count': sql },
        select_params=(contenttype.pk,)
    )