Question

I’m using Django-nonrel on Google App Engine and have the following models (they are simplified).

class Author(models.Model):
    name = models.CharField()

class Book(models.Model):
    author = models.ForeignKey(Author)
    title = models.CharField()

So, if I query like this,

books = Book.objects.all()

and pass the books to a template, and display them like below,

<ul>
{% for book in books %}
   <li>{{ book.title }}{{ book.author.name }}</li>
{% endfor %}
</ul>

I see bunch of datastore.get at the AppStats because of book.author.name. I thought I should use select_related() when I query books, but obviously django nonrel doesn’t support since there is no JOIN on Google App Engine.

Could anyone teach me how I should deal with this kind of scenario? Should I consider denormalizing my models? It would be appreciated if you tell me how you work with it.

Thanks,
Yoo

Answer 1

I’ve struggled to find a way of get_value_from_datastore in Django-Nonrel as Abdul said for a few days. But no luck. So I finally asked @wkornewald at Twitter, then he pointed me to use model.<foreignkey>_id.

Now, I ported Nick’s solution to Django-nonrel. I’m relatively new to Python, so this might not be good coding, but works as I expect.

def prefetch_refprop(entities, prop_id, prop, filter):
    ref_ids = [getattr(x, prop_id) for x in entities]
    ref_entities = dict((x.id, x) for x in filter(id__in=ref_ids))
    for entity, ref_id in zip(entities, ref_ids):
        setattr(entity, prop, ref_entities[ref_id])
    return entities   

#Usage
books = Book.objects.filter(...)
prefetch_refprop(books, 'author_id', 'author', getattr(Author.objects, 'filter'))

Thanks everyone.
Yoo