Question

I’m new to Google Apps and I’ve been messing around with the hello world app that is listed on the google app site. Once I finished the app, I decided to try to expand on it. The first thing I added was a feature to allow the filtering of the guestbook posts by the user that submitted them.

All I have changed/added is simply a handler to the WSGIApplication and a new class for this handler. The model is the same, I’ll post for reference:

class Greeting(db.Model):
    author = db.UserProperty()
    content = db.StringProperty(multiline = True)
    date = db.DateTimeProperty(auto_now_add=True)

Using the Django template I changed the line that displays the authors nickname from:

<b>{{ greeting.author.nickname }}</b> wrote:

to:

<a href="/authorposts/{{ greeting.author.user_id }}">
  {{ greeting.author.nickname }}
</a></b> wrote:

The issue I’m having is that inside my python code I cannot access “greeting.author.nickname”, or any of the other properties, such as “user_id”. However they are accessible from the Django template, as the code I listed above for the template works and correctly creates a link to the author as well as displaying the nickname.

I am using URL Mapping based on the authors (a UserProperty) property “user_id”. I am trying to find a way that I can filter the datastore using the user_id as the criteria. I’ve tried directly applying a filter to the query, I’ve tried pulling all the records and then iterating through them and use an If…Else clause.

I know the value is store into the datastore, because the Django template shows it, what do I have to do to use it as filter criteria?

Answer 1

When querying the Greeting model, you cannot filter on fields within Greeting.author (e.g., greeting.author.nickname). In SQL, this would be done by doing a join on the Greeting and User tables. However, in GAE you can only query properties directly included on the model you are querying.

Since author is a db.UserProperty, you can filter by user like this:

# fetch up to 10 greetings by the current user
user = users.get_current_user()
results = Greeting.all().filter('author =', user).fetch(10)

To filter on other fields within author, you would need to denormalize your Greeting model – i.e., add copies of fields in author which you want to be able to filter Greeting on. For example, if you wanted to filter by author.nickname, you would add an author_nickname field to your Greeting model (and keep its value up to date with author.nickname):

class Greeting(db.Model):
    author = db.UserProperty()
    author_nickname = db.StringProperty() # denormalization (copy of author.nickname)
    content = db.StringProperty(multiline = True)
    date = db.DateTimeProperty(auto_now_add=True)

If you add this sort of denormalization to your model, it might be helpful to use Nick Johnson’s aetycoon library to make author_nickname update whenever author is updated (just so you don’t have to manually enforce that relationship).