Adam Alton Web Developer

The stack:

• django (non-rel)
• Google AppEngine
• djangooappengine

A Datastore cursor is just a marker to a starting point in query.  It’s just like giving an OFFSET to a SQL query, but it’s much more efficient because it allows the datastore to jump to the starting point, instead of having to trawl its way through the first <OFFSET> objects before it starts.

Remember: cursors are an AppEngine Datastore thing.  Models are a Django thing.  But djangoappengine has glued the two together for us with 2 functions:

• get_cursor – give it a Django queryset and it returns the Datastore cursor which marks the position of the end of that query.
• set_cursor – give it a Django queryset and a cursor and it returns a new queryset which will return its results starting at the object marked by the cursor.

Let’s write some code


#views.py
from djangoappengine.db.utils import get_cursor, set_cursor def my_page(request):
""" View some objects. """
results_per_page = 100
queryset = MyModel.objects.all()
#Note, it hasn't called the datastore yet as querysets are lazy
cursor = request.GET.get('cursor')
if cursor:
queryset = set_cursor(queryset, cursor)
results =queryset[0:results_per_page] #starts at the offset marked by the cursor
cursor_for_next_page = get_cursor(results)
template_vars = {
"results": results,
"cursor_for_next_page": cursor_for_next_page,
}
return render_to_response("template.html", template_vars)


#template.html
<h1>Page 1</h1>
<a href="/myview/?cursor={{cursor_for_next_page}}">Next page</a>
{% for object in page1 %}
{{object}} {#whatever you're displaying #}
{% endfor %}

This is fine when you just want a ‘next’ link, but what if you want to be able to skip to page 5 or page 10? Then it gets a bit more tricky.

It’s time for me to eat some dinner, but if this post gets some comments then I’ll expand on this stuff a bit more and reveal ways of making pagination links to other pages (including the previous page).

This entry was posted on Sunday, August 21st, 2011 at 5:58 pm. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.