Django 查询集'；s'count'在'extra'之后是错误的`

当我在Django查询集（称之为

qs

）上以某种方式使用

extra

时，

qs.count（）

的结果不同于

len（qs.all（））

。复制：

创建一个空的Django项目和应用程序，然后添加一个普通模型：

class Baz(models.Model):
    pass

现在制作几个对象：

>>> Baz(id=1).save()
>>> Baz(id=2).save()
>>> Baz(id=3).save()
>>> Baz(id=4).save()

使用

extra

方法仅选择其中的一些会产生预期的计数：

>>> Baz.objects.extra(where=['id > 2']).count()
2
>>> Baz.objects.extra(where=['-id < -2']).count()
2

我认为问题与

django.db.models.sql.query.BaseQuery.get\u count（）

有关。它检查BaseQuery的

select

或

aggregate\u select

属性是否已设置；如果是，则使用子查询。但是

django.db.models.sql.query.BaseQuery.add\u extra

只添加到BaseQuery的

extra

属性，而不是

select

或

aggregate\u select

我怎样才能解决这个问题？我知道我可以只使用

len（qs.all（））

，但如果能够将

额外的
'ed查询集传递给代码的其他部分就好了，这些部分可以调用
count（）
，而不知道它已损坏。
重新定义
get\u count（）
和monkeypatching似乎可以解决问题：

def get_count(self):
    """
    Performs a COUNT() query using the current filter constraints.
    """
    obj = self.clone()
    if len(self.select) > 1 or self.aggregate_select or self.extra:
        # If a select clause exists, then the query has already started to
        # specify the columns that are to be returned.
        # In this case, we need to use a subquery to evaluate the count.
        from django.db.models.sql.subqueries import AggregateQuery
        subquery = obj
        subquery.clear_ordering(True)
        subquery.clear_limits()

        obj = AggregateQuery(obj.model, obj.connection)
        obj.add_subquery(subquery)

    obj.add_count_column()
    number = obj.get_aggregation()[None]

    # Apply offset and limit constraints manually, since using LIMIT/OFFSET
    # in SQL (in variants that provide them) doesn't change the COUNT
    # output.
    number = max(0, number - self.low_mark)
    if self.high_mark is not None:
        number = min(number, self.high_mark - self.low_mark)

    return number

django.db.models.sql.query.BaseQuery.get_count = quuux.get_count

测试：

>>> Baz.objects.extra(select={'negid': '0 - id'}, where=['"negid" < -2']).count()
2

但是，我不确定此修复是否会产生其他意外后果。
验证了Django 1.2.1中仍然存在此问题。
>>> Baz.objects.extra(select={'negid': '0 - id'}, where=['"negid" < -2']).count()
2

def basequery_get_count(self, using):
    """
    Performs a COUNT() query using the current filter constraints.
    """
    obj = self.clone()
    if len(self.select) > 1 or self.aggregate_select or self.extra:
        # If a select clause exists, then the query has already started to
        # specify the columns that are to be returned.
        # In this case, we need to use a subquery to evaluate the count.
        from django.db.models.sql.subqueries import AggregateQuery
        subquery = obj
        subquery.clear_ordering(True)
        subquery.clear_limits()

        obj = AggregateQuery(obj.model)
        obj.add_subquery(subquery, using=using)

    obj.add_count_column()
    number = obj.get_aggregation(using=using)[None]

    # Apply offset and limit constraints manually, since using LIMIT/OFFSET
    # in SQL (in variants that provide them) doesn't change the COUNT
    # output.
    number = max(0, number - self.low_mark)
    if self.high_mark is not None:
        number = min(number, self.high_mark - self.low_mark)

    return number
models.sql.query.Query.get_count = basequery_get_count