Evaluate a readonly field upon creating a new object in Django 1.11 admin

Question

Assuming a the following model

# models.py
class Person(models.Model):
    name = models.CharField(max_length=40)
    birthdate = models.DateField()
    age = models.CharField(max_length=3) # dont mind the type, this is just an example :)

The desired behavior would be to have the age field hidden while the user creates a new Person object, and evaluate its value when the user submits the form. Also, the age field should be visible when views the object instance in admin.

To achieve this, I created a ModelAdmin with a custom ModelForm as follows

# admin.py
from django.contrib import admin
from django import forms
from .models import Person
from dateutil.relativedelta import relativedelta
import datetime

class PersonForm(forms.ModelForm):
    def clean(self):
        cleaned_data = super(PersonForm, self).clean()
        birthdate = cleaned_data.get('birthdate')
        cleaned_data['age'] = relativedelta(datetime.date.today(), birthdate).years
        return cleaned_data

class PersonAdmin(admin.ModelAdmin):
    form = PersonForm
    exclude = ('age',)

admin.site.register(Person, PersonAdmin)

However, after submitting the form, the age field does not get populated, possibly because it is excluded. If I remove the exclude then it works, but the form displays the age field which is not the desired behavior.

You should consider whether you really need an age field at all. It would need to somehow change on every birthday. This would be better provided via a method or property that calculates the current age from the birthdate when required. — Daniel Roseman, May 31 '18 at 09:07

neverwalkaloner · Accepted Answer · 2018-05-31T09:41:11.397

Instead of custom form you can use ModelAdmin's save_model method:

 class PersonAdmin(admin.ModelAdmin):

    def save_model(self, request, obj, form, change):
        birthdate = form.cleaned_data.get('birthdate')
        age = relativedelta(datetime.date.today(), birthdate).years 
        obj.age = age
        super().save_model(request, obj, form, change)

    def get_exclude(self, request, obj=None):
        if not obj:
            return ('age',)
        return self.exclude

Also you probably can use readonly_fields:

 class PersonAdmin(admin.ModelAdmin):
    readonly_fields = ('age',)

    def save_model(self, request, obj, form, change):
        birthdate = form.cleaned_data.get('birthdate')
        age = relativedelta(datetime.date.today(), birthdate).years 
        obj.age = age
        super().save_model(request, obj, form, change)

Thank you. The `age` field is not visible when viewing the object instance. I will edit my question to include this. — Demetris, May 31 '18 at 09:07
@Demetris correct, form validations will not affect `age` field. — neverwalkaloner, May 31 '18 at 09:52

Davit Tovmasyan · Answer 2 · 2018-05-31T09:27:01.760

Or you can hide the input from form.

# admin.py
from django.contrib import admin
from django import forms
from .models import Person
from dateutil.relativedelta import relativedelta
import datetime

class PersonForm(forms.ModelForm):
    age = forms.CharField(widget=forms.HiddenInput, required=False)
    def clean(self):
        cleaned_data = super(PersonForm, self).clean()
        birthdate = cleaned_data.get('birthdate')
        cleaned_data['age'] = relativedelta(datetime.date.today(), birthdate).years
        return cleaned_data

You can use that form only when object is none (create case).

class PersonAdmin(admin.ModelAdmin):
    def get_form(self, request, obj=None, **kwargs):
        if not obj:
            kwargs['form'] = PersonForm
        return super().get_form(request, obj, **kwargs)

admin.site.register(Person, PersonAdmin)

Wouldn't this also hide it when viewing/editing the object instance in admin? — Demetris, May 31 '18 at 09:19

Evaluate a readonly field upon creating a new object in Django 1.11 admin

2 Answers2