Showing custom model validation exceptions in the Django admin site

Question

I have a booking model that needs to check if the item being booked out is available. I would like to have the logic behind figuring out if the item is available centralised so

Answer 1

I've also tried to solve this and there is my solution- in my case i needed to deny any changes in related_objects if the main_object is locked for editing.

1) custom Exception

class Error(Exception):
    """Base class for errors in this module."""
    pass

class EditNotAllowedError(Error):
    def __init__(self, msg):
        Exception.__init__(self, msg)

2) metaclass with custom save method- all my related_data models will be based on this:

class RelatedModel(models.Model):
    main_object = models.ForeignKey("Main")

    class Meta:
        abstract = True

    def save(self, *args, **kwargs):
        if self.main_object.is_editable():
            super(RelatedModel, self).save(*args, **kwargs)
        else:
            raise EditNotAllowedError, "Closed for editing"

3) metaform - all my related_data admin forms will be based on this (it will ensure that admin interface will inform user without admin interface error):

from django.forms import ModelForm, ValidationError
...
class RelatedModelForm(ModelForm):
    def clean(self):
    cleaned_data = self.cleaned_data
    if not cleaned_data.get("main_object")
        raise ValidationError("Closed for editing")
    super(RelatedModelForm, self).clean() # important- let admin do its work on data!        
    return cleaned_data

To my mind it is not so much overhead and still pretty straightforward and maintainable.