list_display should be able to contain sortable references to annotated fields

[Paul McLanahan]

Overriding the queryset method of a ModelAdmin should be an easy way to annotate the QuerySet used in the admin list view for a model. My use case is that I have two IntegerFields containing counts of various things, but what I'd like to be able to display and sort by in the admin list view is the sum of these two fields. This can best be explained by an example:

# Given this model
class Article(models.Model):
    title = models.CharField(max_length=255)
    num_a = models.PositiveIntegerField()
    num_b = models.PositiveIntegerField()

# Want ModelAdmin like...
class ArticleAdmin(admin.ModelAdmin):
    list_display = ('title' 'total_nums')
    
    def queryset(self, request):
        qs = super(ArticleAdmin, self).queryset(request)
        return qs.extra(select={'total_nums':'num_a + num_b'})

This fails at model validation with a ImproperlyConfigured exception in django.contrib.admin.validation.validate because the model has no 'total_nums' field, which we know. But the model validation mechanism has no access to the instance of the ModelAdmin, and therefore no access to the queryset to be able to check for any extras or annotations.

I tried fixing this and would have submitted a patch, but I failed in the time I had. However, there is a workaround I discovered and am using, but it seems silly. You change ArticleAdmin to the following:

class ArticleAdmin(admin.ModelAdmin):
    list_display = ('title' 'total')
    
    def total(self, obj):
        return obj.total_nums
    total.admin_order_field = 'total_nums'
    
    def queryset(self, request):
        qs = super(ArticleAdmin, self).queryset(request)
        return qs.extra(select={'total_nums':'num_a + num_b'})

This seems overly verbose and a little hacky, but it does work. I'd say that makes this ticket non-urgent, though I do wonder how many developers gave up before discovering this technique.

[Simon Meers]

We don't really want to encourage the use of extra in general, however making additional queryset attributes available in list_display could be quite useful when used in conjunction with annotate.

[Aymeric Augustin]

Change UI/UX from NULL to False.

[Aymeric Augustin]

Change Easy pickings from NULL to False.

[Jon Culver]

I had a use case specific to counts, so I took a crack at addressing this — and it's my first contribution, so feedback is wholly welcomed and appreciated.

​https://github.com/bahoo/django/compare/django:stable/1.7.x...ticket_14336_1_7?expand=1

This adds list_annotate which can be populated with any aggregate to a related object or ManyToMany field, though Count is the only pertinent one here, I suspect.

with models.py

class Programmer(models.Model):
    pass

class Software(models.Model):
    title = models.CharField(max_length=30)
    contributors = models.ManyToManyField('Programmer')

class Commit(models.Model):
    project = models.ForeignKey(Software, related_name='commits')
    author = models.ForeignKey(Programmer)

and admin.py

from django.db import Count
from django.contrib import admin
from .models import Software

@admin.register(Software)
class SoftwareAdmin(admin.ModelAdmin):
    list_display = ('title', 'contributors__count', 'commits__count')
    list_annotate = [Count('contributors'), Count('commits')]

'Contributors count' and 'Commits count' become two sortable columns in the changelist admin.

I'm happy to add the above example to the documentation and submit a pull request, but could I trouble someone for some feedback on my approach? Thank you!

[Tim Graham]

I'm removing consideration of "extra" fields from the summary because QuerySet.extra() is now discouraged and likely to be deprecated in the future.

#27229 is a related ticket that suggests allowing aggregations in list_display. I hope to figure out a suitable path forward for this on the ​django-developers mailing list thread. Jon, sorry you didn't get any feedback from your original work. Feel free to chime in here if you still have time and interest in working on this.

[Jon Culver]

Sure, I'd love to help. I'll have more free time to devote to it come mid-November, would that work? Thanks, Tim.