Django - create an admin list filter on a calculated value

admin.py

from django.contrib import admin

from .filters import CalculatedFieldFilter
from .models import Item

@admin.register(Item)
class ItemAdmin(admin.ModelAdmin):
    list_display = ('val', 'calculated_field')
    list_filter = (CalculatedFieldFilter, )

calculated-admin-list-filter.md

Create an admin list filter on a calculated value

What

Filter a list in the Django admin based on a calculated field

Why

The Django admin list_filter option only accepts model field names. You cannot filter based on a model method or callable, although you can display the value of a model method in the list_display option. If you want to filter by a calculated value, you will need to use a custom filter that queries the database for the objects you want.

Why Not

You may be better off creating a non-editable database field, overriding the save() method to populate that field, and then using that field in the default list_filter.

How

Create a custom Django admin list filter that implements the logic of your calculated field

• Create your model
• Define a custom list filter that queries the database according to whatever logic you need
• Add your custom filter to the list_filter option of your admin class

filters.py

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

class CalculatedFieldFilter(admin.SimpleListFilter):
    title = 'calculated value' # this is shown in the admin as the filter heading. Ex. "By calculated value"
    parameter_name = 'calculated_value' # this is the parameter used in the admin URL along with the lookups below. Ex. calculated_value=1

    def lookups(self, request, model_admin):
        # these are two-tuples of (parameter_value, human-readable_option_name)
        return (
            (1, 'The answer'),
            (0, 'Not the answer'),
        )
    
    def queryset(self, request, queryset):
        # this mirrors the logic in our models calculated_field method, but performs the operation in the database instead of in python
        lookup_condition = models.Q(val=42)
        if self.value() == 1:
            return queryset.filter(lookup_condition)
        
        if self.value() == 0:
            return queryset.exclude(lookup_condition)

models.py

from django.db import models

class Item(models.Model):
    val = models.IntField()
    
    def calculated_field(self):
        if self.val == 42:
            return 'The answer'
        else:
            return 'Not the answer'