Django: Faking a field in the admin interface?

Question

I have a model, Foo. It has several database properties, and several properties that are calculated based on a combination of factors. I would like to present these

Answer 1

I would suggest you subclass a modelform for Foo (FooAdminForm) to add your own fields not backed by the database. Your custom validation can reside in the clean_* methods of ModelForm.

Inside the save_model method of FooAdmin you get the request, an instance of Foo and the form data, so you could do all processing of the data before/after saving the instance.

Here is an example for a model with a custom form registered with django admin:

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


class Foo(models.Model):
    name = models.CharField(max_length=30)


class FooAdminForm(forms.ModelForm):
    # custom field not backed by database
    calculated = forms.IntegerField()

    class Meta:
        model = Foo 


class FooAdmin(admin.ModelAdmin):
    # use the custom form instead of a generic modelform
    form = FooAdminForm

    # your own processing
    def save_model(self, request, obj, form, change):
        # for example:
        obj.name = 'Foo #%d' % form.cleaned_data['calculated'] 
        obj.save()


admin.site.register(Foo, FooAdmin)

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Providing initial values for custom fields based on instance data

(I'm not sure if this is the best solution, but it should work.)

When a modelform for a existing model instance in the database is constructed, it gets passed this instance. So in FooAdminForm's __init__ one can change the fields attributes based on instance data.

    def __init__(self, *args, **kwargs):
        # only change attributes if an instance is passed            
        instance = kwargs.get('instance')
        if instance:
            self.base_fields['calculated'].initial = (instance.bar == 42)
        forms.ModelForm.__init__(self, *args, **kwargs)