Monmonja Programming Blog

So you want to implement auto increment on your datastore? Here is a simple tutorial of how to do it on App Engine. This auto increment will use integers, this does not cover more advance auto increments one like UUID.

For our example we would use this datastore model:
class Post(db.Model):
title = db.StringProperty()
body = db.StringProperty()


Then you would save records like with something like
post = Post()
post.title = "Example"
post.body = "This is the body"
post.put()


Then you shall see this saved on the datastore. But where did i declared the id? App engine by default will give each record a Key, having said that each row by default will have an auto incremented integer. To retrive a row by the id you would do something like
currentPost = Post.get_by_id(<id of the post>)

What if i fetch from the model, how can i get the id?
query = datamodel.Post().all()
for result in query:
self.response.out.write(result.key().id())


How can i get my auto generated id on templates?
{% for post in posts %}
{{ post.key.id }}
{% endfor %}


App Engine by default uses django, a Python web framework that’s a bit like Rails or CakePHP, most of the tutorial on the App Engine Documentation would not teach you to use Templates except the main Using Templates part. While this tutorial is good, it would not teach you how have a more optimized version. Here is how i did, i suggest you to read the App Engine Using Templates Tutorial:

import os
from google.appengine.ext import webapp
from google.appengine.ext.webapp import template
def page_display(page,file,values={}):
template_values = {}
template_values.update(values)
try:
template_values.update(page.template_values)
except:
pass
path = os.path.join(os.path.dirname(__file__),file)
page.response.out.write(template.render(path,template_values))
class MainPage(webapp.RequestHandler):
def get(self,page):
self.template_values = {
"message" : "Hello World",
"message2" : "Hello"
}
page_display(self,"templates/index.html")
def main():
application = webapp.WSGIApplication(
[
('/', MainPage)
],
debug = True)
wsgiref.handlers.CGIHandler().run(application)
if __name__ == "__main__":
main()

As you can see if you have a lot of classes it would be a easier to remember and use
page_display(self,"templates/index.html")
then
path = os.path.join(os.path.dirname(__file__), 'index.html')
self.response.out.write(template.render(path, template_values))