Question

I have the following model classes:

class Collection(db.Model):
  name = db.StringProperty()
  text_keys = db.ListProperty(db.Key)

class Text(db.Model):
  name = db.StringProperty()
  content = db.StringProperty()

and I’m trying to do the following:

class Collections(webapp.RequestHandler):
  def get(self):
    collections = model.Collection.all() # works fine

    for c in collections:
      c.number_of_texts = len(c.text_keys) # does not work

    template_values = {
      'collections': collections,
    }

I’m certainly no python-expert, but shouldn’t this work?

UPDATE:

By does not work i mean that the variable number_of_texts is not added to the model object.

In my django-template, the following code generates nothing, except for the collection name:

{% for c in collections %}
<p>{{c.name}}, number of texts: {{c.number_of_texts}}</p>
{% endfor %}

SOLUTION:

Thanks to RocketDonkey for pointing out that this can be done in a more elgant fashion using django formatting:

{% for c in collections %}
<p>{{c.name}}, number of texts: {{c.text_keys|length}}</p>
{% endfor %}

or by passing a separate dictionary with the names and lenghts to the template, if a similar problem with no good formatting solution should occur.

Answer 1

So it appears that you are trying to write to the number_of_texts property of the Collection model (which does not exist 🙂 ). If you just need to get the number of items in that list element, you’ll need to store it in a separate variable not tied to c:

for c in collections:
  number_of_texts = len(c.text_keys)

In order to add the length of the list to your document (assuming you don’t need it anywhere else), try using the length function in your template:

{% for c in collections %}
    <p>{{c.name}}, number of texts: {{c.text_keys|length}}</p>
{% endfor %}

This may not work depending on your templating engine (I have only used one so I’m far from an expert), but it will hopefully get you what you want.