计算Django的平均年龄 [英] Calculating average age in Django

问题描述

我需要找出商品的平均年龄（按各种标准分组，但这似乎不是问题）。但是我找不到如何使用Django（在MySQL之上）有效地在DateTimeField上创建聚合的方法。

I need to find out average age of items (groupped by various criteria, but it does not seem to be the issue). However I fail to find how to effectively create aggregation over DateTimeField using Django (on top of MySQL).

纯 Item.objects.all.aggregate （Avg（'created'））似乎会产生绝对虚假的值（例如20081988007238.133）并使用 Item.objects.all.extra（...）我还没有找到一种汇总方法。

Pure Item.objects.all.aggregate(Avg('created')) seems to produce absolutely bogus values (eg. 20081988007238.133) and using Item.objects.all.extra(...) I have not found a way how to aggregate.

当然，我可以手动创建SQL查询（例如 SELECT AVG（UNIX_TIMESTAMP（创建））FROM items_item ），但我宁愿避免在应用程序中使用MySQL特定代码。

Of course I can manually create SQL query (something like SELECT AVG(UNIX_TIMESTAMP(created)) FROM items_item), but I'd prefer to avoid using MySQL specific code in the application.

仅供参考，示例我用于测试的模型：

Just for the reference, sample model I use for testing:

class Item(models.Model):
    name = models.CharField(max_length = 255)
    created = models.DateTimeField()

推荐答案

我认为没有其他方法可以使用 extra 方法。然后应该看起来像这样（未经测试）：

I think there is no other way as using the extra method. It should look like this then (not tested):

Item.objects.extra('avg': 'AVG(UNIX_TIMESTAMP(created))'.values('avg')

问题是，您必须转换日期要早于UNIX时间戳记。否则，您将无法计算平均值。字符串的平均值确实会产生垃圾。

The problem is, that you have to convert the date to a UNIX timestamp before. Otherwise you cannot calculate the average. An average of string will indeed produce garbage.

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