Concurrent edit prevention through forms

forms.py

from django import forms
from django.core.exceptions import ValidationError

from .models import Foo


class VersionNumberFormMixin(forms.Form):
    version_number = forms.IntegerField(min_value=0, widget=forms.HiddenInput)

    def __init__(self, *args, **kwargs):
        super(VersionNumberFormMixin, self).__init__(*args, **kwargs)
        if not self.instance.pk:
            # We are working with a new object so force pre-initial version
            self.instance.version_number = 0
        self.initial['version_number'] = self.instance.version_number

    def clean_version_number(self):
        submitted = self.cleaned_data.get('version_number', 0)
        given = self.instance.version_number
        if given != submitted:
            raise ValidationError('Invalid version', code='invalid')
        return submitted


# Use it like this:

class FooForm(VersionNumberFormMixin, forms.ModelForm):
    class Meta:
        model = Foo
        fields = ('name',)

models.py

from django.db import models


class VersionNumberMixin(models.Model):
    version_number = models.PositiveIntegerField()

    class Meta:
        abstract = True

    def save(self):
        self.version_number += 1
        super(VersionNumberMixin, self).save()


# Use it like this:

class Foo(VersionNumberMixin, models.Model):
    name = models.CharField(max_length=20)