What's the best way to model polls? - google-app-engine

Question

The domain is like this:
class Poll(db.Model):
question = db.StringProperty()
...
class Choice(db.Model):
poll = db.ReferenceProperty(Poll)
choice = db.StringProperty()
class Vote(db.Model):
user = db.ReferenceProperty(User)
choice = db.ReferenceProperty(Choice)
(This is not actually a definitive model, its just pseudo-diagram)
The things I need to query are:
Total number of votes for each poll on screen
Total number of votes for each option for each poll on screen
If the current user voted, for each poll
I have come up with some other schema using shared counters, list properties and none (with my intrinsic limitations) seems to be working. Oh, and of course, it needs to be super fast :)
Could you help me model my data?
Thank you
edit: Thanks to #Nick Johnson I can make a more accurate description of my problem, he suggested this schema
class Poll(db.Model):
question = db.StringProperty(indexed=False, required=True)
choices = db.StringListProperty(indexed=False, required=True)
votes = db.ListProperty(int, indexed=False, required=True)
class Vote(db.Model):
# Vote is a child entity of Poll, so doesn't need an explicit reference to it
# Vote's key name is the user_id, so users can only vote once
user = db.ReferenceProperty(User, required=True)
choice = db.IntegerProperty(required=True)
The problem with this, is that I can't query efficiently showing if the user has voted or not on a particular poll. Also, I want this shema to resist to lets say 1MM votes per poll or something (maybe I'd never get there, but I would like to aim there)
To solve this I was thinking of adding an EntityIndex like this:
class PollIndex(db.Model):
# PollIndex is child of Poll
voters = db.ListProperty(db.Key)
voters_choices = db.ListProperty()
# other search parameters
Then when I have to query for a list of polls I can only do it with 2 queries:
# get keys from pollindex where user is not there
# get keys from pollindex where user is there
# grabb all the polls
An other cool thing is that if the voters increase in size I can dinamically add more PollIndexes
What do you think of this approach?

Answer 1

The answer somewhat depends on what you expect the maximum sustained rate of updates to the poll to be. I'll assume initially that it's going to be quite limited (<1 per second typical, with peaks up to 10 per second).
Your design is mostly okay, except for a couple of tweaks:
Don't store choices as a separate entity, just store them as a list on the poll
Keep a running total of votes on the Poll entity for fast retrieval
With those changes, your model looks something like this:
class Poll(db.Model):
question = db.StringProperty(indexed=False, required=True)
choices = db.StringListProperty(indexed=False, required=True)
votes = db.ListProperty(int, indexed=False, required=True)
class Vote(db.Model):
# Vote is a child entity of Poll, so doesn't need an explicit reference to it
# Vote's key name is the user_id, so users can only vote once
user = db.ReferenceProperty(User, required=True)
choice = db.IntegerProperty(required=True)
# Here's how we record a vote
def record_vote(poll_key, user, choice_idx):
# We assume 'user' is an instance of a datastore model, and has a property 'user' that is
# a users.User object
poll = Poll.get(poll_key)
vote = Vote.get_by_key_name(user.user.user_id(), parent=poll)
if vote:
# User has already voted
return
vote = Vote(key_name=user.user.user_id(), parent=poll, user=user)
poll.votes[choice_idx] += 1
db.put([vote, poll])
If you need higher throughput, you should modify the Vote record to not be a child of Poll (and change its key name to incorporate both poll ID and user ID), and then either use write-behind counters with Memcache or a pull queue to aggregate the results into updates to the Poll totals.