Question

from django.db import models
from django.contrib.auth.models import User

class Product(models.Model):
name = models.CharField(max_length = 127)
description = models.TextField()
code = models.CharField(max_length = 30)
lot_no = models.CharField(max_length = 30)
inventory = models.IntegerField()
commited = models.IntegerField()
available = models.IntegerField()
reorder = models.IntegerField()
created_date = models.DateField(auto_now_add = True)
comment_user = models.ForeignKey(User, null=True)
comment_txt = models.TextField()

def __unicode__(self):
    return self.code + " - " + self.name + " - " + self.lot_no + " - " + str(self.created_date)

@property
def available(self):
    return self.inventory - self.commited

Im trying to have available calculated by (inventory – self) when a person enters in the data for inventory and commited in django admin template. But I’m not sure how.

Thanks,
Jon

Answer 1

It seems like you may have two problems; the overlapping available property+field and availability not showing up as you expect in the admin.

Choose one way (property or field) to represent the availability and go with it. Don and Seth have shown a way to do it using a field and Daniel and Ignacio have suggested going with a property.

Since you really want this field to show up in the admin just go with the field; give it a helpful help_text=”…”, remove the @property, and override save().

class Product(models.Model):
    # ...
    availability = models.IntegerField(help_text="(updated on save)")

    # Use Seth's save()
    def save(self, *args, **kwargs):
        self.availability = self.inventory - self.commited
        super(Product, self).save(*args, **kwargs)

This is not the best way to do things in terms of normalized data but it will probably be the simplest solution to your current problem.

If you are using trunk instead of Django-1.1.1, you can also use readonly_fields in the admin.