Sorting by related_id on an admin change-list page

Using Django
1

Please consider some arbitrary model with a foreignkey to another model (in Django 3.2):

class SomeModel(models.Model):
    related = models.ForeignKey(to=OtherModel, ...)

We want to show the related_id (see docs) on the admin change list page, for example:

class SomeModelAdmin(admin.ModelAdmin):
    list_display = ['related_id', ...]

Pretty straightforward, but the problem is: we cannot sort by this related_id column.

Now, there’s at least one way around this, e.g. using admin.display ordering (or the good old admin_order_field):

class SomeModelAdmin(admin.ModelAdmin):
    list_display = ['display_related_id', ...]

    @admin.display(description='related id', ordering='related_id')
    def display_related_id(self, obj):
        return obj.related_id

Another alternative would be to use list_display = ['related', ...], which does allow sorting, then setting OtherModel.__str__ to return the id, but this may have side effects outside the admin.

Anyway, these workarounds do the job, but they are inconvenient, and the question remains:

Why can we not sort by related_id out-of-the-box?

Does it have to do with the fact that related_id is not an actual Field?

2

Answering “why” for questions like this is frequently “difficult to impossible”, because the people who made those decisions aren’t likely here to answer them.

However, I will address your second question - no, related_id is an actual column in the database.

I believe (but my answer here is far from authoritative), that the issue is probably related to list_display allowing the elements of that list to be methods, ModelAdmin attributes and model attributes. That implies to me that they can’t just rely upon a database sort for ordering the list display, and that the elements of the list to be displayed must be sorted within the view generating that list - putting different requirements on identifying how that list is to be sorted.

3

One way to make this a bit easier is to define a function that returns a function with the attributes you’d like.

def display_fk_id(field, ordering):
    @admin.display(ordering=ordering)
    def dynamic_display_column(obj):
        return getattr(obj, field)
    return display_fk_id

class SomeModelAdmin(admin.ModelAdmin):
    list_display = [display_fk_id('related_id', ordering='related_id'), ...]

Edit: I have not tried this, but the idea should work.

4

Thanks, I am aware of that, but what I meant is that it is not a models.fields.Field (related is, but related_id is not, it is the representation of that field in the database).

5

Thanks for your input.

Perhaps I should clarify a little:

The problem is not that I don’t know how to work around the issue: the basic @admin.display() implementation from my example already works well enough.

The problem is that I am actually wondering why the custom display ordering implementation for related_id is necessary in the first place. Why does it not work out-of-the-box, like it does when we do list_display = ['related', ...]?

I just do not see why the sorting would work for related, but not for related_id: There is no ambiguity as to the database field to be used (both use the same one).

So, to rephrase the question:

Why doesn’t sorting by related_id work out-of-the-box? Is this by design, or is it just a feature waiting to be implemented, or is it a bug?

Here’s from the docs:

Usually, elements of list_display that aren’t actual database fields can’t be used in sorting (because Django does all the sorting at the database level).

That does not seem to apply to related_id which is an “actual database field”.