Question

I am overriding the save_model method of my ModelAdmin so I can set some additional fields on the model. However, I keep getting a DoesNotExist exception. I stepped through the code with a debugger and I’m a bit confused.

The model has this field:

created_by = models.ForeignKey(User, db_column='created_by', related_name='%(class)s_creator')

In the save_model method when I look at the attributes of the obj parameter, I see this:

Name:
created_by

Value:
str: Traceback (most recent call last):
  File "/home/vicki/.eclipse/org.eclipse.platform_3.7.0_155965261/plugins/org.python.pydev.debug_2.3.0.2011121518/pysrc/pydevd_resolver.py", line 182, in _getPyDictionary
    attr = getattr(var, n)
  File "/usr/lib/pymodules/python2.7/django/db/models/fields/related.py", line 301, in __get__
    raise self.field.rel.to.DoesNotExist
DoesNotExist

Name:
created_by_id

Value:
NoneType: None

Where did ‘created_by_id’ come from, and why does ‘created_by’ not exist?

Answer 1

Is there a particular reason why you have used db_column='created_by'? Are you dealing with a legacy database? Normally, you would let Django handle the name of the db column. By default, Django will append _id to the field name to create the db column name. This is explained in the Django Foreign Key docs.

On your model instance you can access:

`obj.created_by_id` # the primary key of the related User instance
`obj.created_by` # The `User` instance

You can’t access obj.created_by in your method because you haven’t assigned a user to it yet. The created_by_id value is None, and trying to fetch a User with primary key None naturally raises a DoesNotExist exception.

Your save_model method should look something like this:

class MyModelAdmin(admin.ModelAdmin):
    def save_model(self, request, obj, form, change):
        obj.created_by = request.user
        obj.save()