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Django: Order a model by a many-to-many field

Posted by: admin December 31, 2017 Leave a comment

Questions:

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.

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).

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.

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.

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

Answers:

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')

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

The generated query is:

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

Questions:
Answers:

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

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:

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

Questions:
Answers:

Something like this in SQL:

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.

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.

I don’t know how this interacts with Django.