Can anyone help me with Django admin forms? I'm trying to automatically set a field when it's saved, but it doesn't seem to work.

Can anyone help me with Django admin forms?
I'm trying to automatically set a field when it's saved, but it doesn't seem to work. 
My understanding was to do the following:
```

# models.py

class BlogPost(models.Model):

    author = models.ForeignKey(User, on_delete=models.CASCADE)

    posted_date = models.DateField(default=timezone.now)

    title = models.CharField(max_length=200)

    text = models.TextField(max_length=1000)

# in admin.py

class BlogPostAdmin(admin.ModelAdmin):

    model = BlogPost

    def formfield_for_choice_foreignkey(self, db_field, request, **kwargs):

        if db_field.name == 'author':

            kwargs['queryset'] = User.objects.filter(username=request.user.username)

        return super(BlogPostAdmin, self).formfield_for_foreignkey(db_field, request, **kwargs)

```
Preferably I would like the field to just be hidden when editing or creating a new `BlogPost`.