Django：通过多对多字段订购模型 [英] Django: Order a model by a many-to-many field
问题描述
我正在编写一个具有人物模型的Django应用程序，并且我遇到了一个障碍。我将角色对象分配给使用多对多关系的角色 - 角色具有名称和权重。我希望以最重的角色重新命令我的名单。如果我做People.objects.order_by（' - roles__weight'），那么当人们分配了多个角色时，我会得到重复的。
I am writing a Django application that has a model for People, and I have hit a snag. I am assigning Role objects to people using a Many-To-Many relationship - where Roles have a name and a weight. I wish to order my list of people by their heaviest role's weight. If I do People.objects.order_by('-roles__weight'), then I get duplicates when people have multiple roles assigned to them.
我的初始想法是添加一个称为最重的角色权重的非规范化字段，并按此排序。每次从用户添加或删除新角色时，都可以对其进行更新。但是，事实证明，每次在Django中更新ManyToManyField时，都无法执行自定义操作（ yet ，无论如何）。
My initial idea was to add a denormalized field called heaviest-role-weight - and sort by that. This could then be updated every time a new role was added or removed from a user. However, it turns out that there is no way to perform a custom action every time a ManyToManyField is updated in Django (yet, anyway).
所以，我以为我可以完全超越，写一个自定义字段，描述符和管理器来处理这个 - 但这似乎是非常当ManyToManyField动态创建ManyRelatedManager时，很难。
So, I thought I could then go completely overboard and write a custom field, descriptor and manager to handle this - but that seems extremely difficult when the ManyRelatedManager is created dynamically for a ManyToManyField.
我一直在想出一些可以为我做的这个聪明的SQL - 我确信这是可能的有一个子查询（或几个），但是我会担心Django支持的所有数据库后台都不能兼容。
I have been trying to come up with some clever SQL that could do this for me - I'm sure it's possible with a subquery (or a few), but I'd be worried about it not being compatible will all the database backends Django supports.
有没有人这样做过 - 或有任何想法可以实现吗？
Has anyone done this before - or have any ideas how it could be achieved?
推荐答案
Django 1.1（目前的测试版）添加了聚合支持。您的查询可以通过以下方式完成：
Django 1.1 (currently beta) adds aggregation support. Your query can be done with something like:
from django.db.models import Max
People.objects.annotate(max_weight=Max('roles__weight')).order_by('-max_weight')
生成的查询是：
The generated query is: $ c> SELECT people.id，people.name，MAX（role.weight）AS max_weight
FROM people LEFT OUTER JOIN people_roles ON（people.id = people_roles.people_id）
LEFT OUTER JOIN角色ON（ people_roles.role_id = role.id）
GROUP BY people.id，people.name
ORDER BY max_weight DESC
SELECT people.id, people.name, MAX(role.weight) AS max_weight
FROM people LEFT OUTER JOIN people_roles ON (people.id = people_roles.people_id)
LEFT OUTER JOIN role ON (people_roles.role_id = role.id)
GROUP BY people.id, people.name
ORDER BY max_weight DESC
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