Django：通过多对多字段订购模型

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.

我正在编写一个Django应用程序，它有一个People模型，我遇到了麻烦。我正在为使用多对多关系的人分配角色对象 - 其中角色具有名称和权重。我希望按照他们最重要的角色来命令我的人员名单。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。

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).

我最初的想法是添加一个称为最重角色权重的非规范化字段 - 并按此排序。然后，每次从用户添加或删除新角色时，都可以更新此更新。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（无论如何）。

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.

所以，我认为我可以完全超越并编写一个自定义字段，描述符和管理器来处理这个问题 - 但是当为ManyToManyField动态创建ManyRelatedManager时，这似乎非常困难。

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.

我一直试图想出一些聪明的SQL，可以为我做这个 - 我敢肯定它可能有一个子查询（或一些），但我担心它不兼容所有的数据库后端Django支持。

Has anyone done this before - or have any ideas how it could be achieved?

有没有人以前做过这个 - 或者有什么想法可以实现它？

3 个解决方案

#1

Django 1.1 (currently beta) adds aggregation support. Your query can be done with something like:

Django 1.1（目前为测试版）增加了聚合支持。您的查询可以通过以下方式完成：

from django.db.models import Max
People.objects.annotate(max_weight=Max('roles__weight')).order_by('-max_weight')

This sorts people by their heaviest roles, without returning duplicates.

这会按照最重要的角色对人进行排序，而不会返回重复项。

The generated query is:

生成的查询是：

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

#2

Here's a way to do it without an annotation:

这是一种没有注释的方法：

class Role(models.Model):
    pass

class PersonRole(models.Model):
    weight = models.IntegerField()
    person = models.ForeignKey('Person')
    role = models.ForeignKey(Role)

    class Meta:
        # if you have an inline configured in the admin, this will 
        # make the roles order properly 
        ordering = ['weight'] 

class Person(models.Model):
    roles = models.ManyToManyField('Role', through='PersonRole')

    def ordered_roles(self):
        "Return a properly ordered set of roles"
        return self.roles.all().order_by('personrole__weight')

This lets you say something like:

这可以让你说：

>>> person = Person.objects.get(id=1)
>>> roles = person.ordered_roles()

#3

Something like this in SQL:

SQL中的这样的东西：

select p.*, max (r.Weight) as HeaviestWeight
from persons p
inner join RolePersons rp on p.id = rp.PersonID
innerjoin Roles r on rp.RoleID = r.id
group by p.*
order by HeaviestWeight desc

Note: group by p.* may be disallowed by your dialect of SQL. If so, just list all the columns in table p that you intend to use in the select clause.

注意：您的SQL方言可能不允许使用p。*分组。如果是这样，只需列出要在select子句中使用的表p中的所有列。

Note: if you just group by p.ID, you won't be able to call for the other columns in p in your select clause.

注意：如果您只是按p.ID分组，则无法在select子句中调用p中的其他列。

I don't know how this interacts with Django.

我不知道这是如何与Django交互的。

#1