Question

I’m writing a Django page in Google App Engine where users can create or edit an instance of an object and save it. Because the code for creating a new object and editing an existing one is (almost) identical, I’m using the same code for both edit and create. The object is called Group.

At the top of the view function I load the Group instance if an id was provided, or i create a new one if an id was not provided:

def group_edit(request, group_id=None):

    if(group_id is None):
        group_inst = Group()
    else:
        group_inst = db.get(db.Key.from_path('Group', int(group_id)))

        if(group_inst is None):
            raise Http404()

The rest of the view function then operates on group_inst without knowing or caring whether group_inst is a new instance or an existing one. There is one place where I need to know the difference, however: the page title. If the user is creating a new Group, I want to display “create group”, otherwise I want to display “edit group – {{ group.name }}”

Intuitively, I would just check in the template if the object’s key is None:

<h1>
    {% if group_inst.key %}
        Edit Group - {{ group_inst.name }}
    {% else %}
        Create Group
    {% endif %}
</h1>

However this raises a NotSavedError. Looking through the GAE docs i don’t see anything helpful. How can I check whether the instance has a corresponding datastore record or not?

Answer 1

You should use has_key() rather than just accessing the key:

{% if group_inst.has_key %}