Working with Many-to-Many Relationships in Django Admin

The issue you're encountering is related to how Django handles many-to-many relationships in the admin interface. Let me explain the problem and provide a solution.

Understanding the Issue

The filter_horizontal and filter_vertical widgets only work on the model that contains the actual ManyToManyField definition. In your case, the Study model has the groups field, so you can use filter_horizontal there.

However, on the Group side, there's no explicit ManyToManyField - instead, Django creates a reverse relation called study_set. This reverse relation doesn't support the filter widgets directly, which is why you're getting errors.

Solution: Create a Custom Form for GroupAdmin

To achieve what you want, you need to create a custom form for the Group admin that includes a proper ManyToManyField for studies:

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# admin.py
from django import forms
from django.contrib import admin
from django.contrib.auth.admin import GroupAdmin as BaseGroupAdmin
from django.contrib.auth.models import Group
from .models import Study

class GroupAdminForm(forms.ModelForm):
    studies = forms.ModelMultipleChoiceField(
        queryset=Study.objects.all(),
        required=False,
        widget=admin.widgets.FilterHorizontalWidget()
    )

    class Meta:
        model = Group
        fields = '__all__'

    def __init__(self, *args, **kwargs):
        super().__init__(*args, **kwargs)
        if self.instance.pk:
            self.initial['studies'] = self.instance.study_set.all()

    def save(self, commit=True):
        group = super().save(commit=commit)
        if commit:
            group.study_set.set(self.cleaned_data['studies'])
        return group

admin.site.unregister(Group)
@admin.register(Group)
class GroupAdmin(BaseGroupAdmin):
    form = GroupAdminForm

This approach:

1. Creates a custom form with a studies field that uses the FilterHorizontalWidget
2. Initializes the field with the current related studies
3. Saves the relationship when the form is saved

Alternative: Modify Your Model Structure

If you prefer a cleaner solution, you could modify your model to have explicit ManyToManyField declarations on both sides:

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# models.py
from django.contrib.auth.models import Group
from django.db import models

class Study(models.Model):
    groups = models.ManyToManyField(
        Group, 
        blank=True,
        related_name='studies'  # This creates an explicit 'studies' attribute
    )

Then in your admin:

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# admin.py
from django.contrib import admin
from django.contrib.auth.admin import GroupAdmin as BaseGroupAdmin
from django.contrib.auth.models import Group
from .models import Study

@admin.register(Study)
class StudyAdmin(admin.ModelAdmin):
    filter_horizontal = ["groups"]

admin.site.unregister(Group)
@admin.register(Group)
class GroupAdmin(BaseGroupAdmin):
    filter_horizontal = ["studies"]  # Now this will work

This approach is cleaner but requires a migration if you've already deployed your application.

The issue with your inline approach is that inlines are designed to show the related objects as individual rows, not as a filter widget. That's why it doesn't provide the two-column filter interface you're looking for.