Weird behavior of a modified_by and created_by field

I have the following model where  created_by or modified_by (user) are defined as CharField.

models.py

class Item(models.Model):
    barcode = models.CharField(primary_key=True, max_length=20)
    description = models.CharField(max_length=255)

    created_date = models.DateTimeField(editable=False, auto_now_add=True,null=True, blank=True) # Field name made lowercase.
    modified_date = models.DateTimeField(editable=False,auto_now=True,null=True, blank=True) # Field name made lowercase.
    created_by = models.CharField(max_length=15,null=True,blank=True)
    modified_by = models.CharField(max_length=15,null=True,blank=True)

AND,

According to what I read in the internet (see http://stackoverflow.com/questions/4754485/dry-way-to-add-created-modified-by-and-time) in admin.py I redefined save_model as follows:

admin.py

class ItemOption(admin.ModelAdmin):
    search_fields=['barcode','description']
    list_display = ('barcode','description','created_by', 'modified_by')
    fields=('barcode','description','created_by', 'modified_by')

    def save_model(self, request, obj, form, change):
        if change:
            obj.modified_by = request.user
            obj.modified_date = datetime.now()
        else:
            obj.created_by = request.user
            obj.created_date = datetime.now()
        obj.save()

Now it happens (see the attached png file) that - when  a record is created in item model - in created_by you "correctly" find the username as a string, while - when  a record is modified in item model - in modified_by you find the pk number of the user (which, by the side, is correct).

Why is that and how can I fix it  in order to have the user as a string in modified_by?

Ciao

Vittorio