Question

I am using appengine with python 2.7 and webapp2 framework. I am not using ndb.model.

I have the following model:

class Story(db.Model);
    name = db.StringProperty()

class UserProfile(db.Model):
    name = db.StringProperty()
    user = db.UserProperty()

class Tracking(db.Model):
   user_profile = db.ReferenceProperty(UserProfile)
   story = db.ReferenceProperty(Story)
   upvoted = db.BooleanProperty()
   flagged = db.BoolenProperty()

A user can upvote and/or flag a story but only once. Hence I came up with the above model.
Now when a user clicks on the upvote link, on the database I try to see if the user has not already voted it, hence I do try to do the following:

1. get the user instance with his id as up = db.get(db.Key.from_path('UserProfile', uid))

2. then get the story instance as follows s_ins = db.get(db.Key.from_path('Story', uid))

3. Now it is the turn to check if a Tracking based on these two exist, if yes then don’t allow voting, else allow him to vote and update the Tracking instance.

What is the most convenient way to fetch a Tracking instance given an id(db.key().id()) of user_profile and story?

What is the most convenient way to save a Tracking model having given a user profile id and an story id?

Is there a better way to implement tracking?

Answer 1

You can try tracking using lists of keys versus having a separate entry for track/user/story:

class Story(db.Model);
  name = db.StringProperty()

class UserProfile(db.Model):
  name = db.StringProperty()
  user = db.UserProperty()

class Tracking(db.Model):
  story = db.ReferenceProperty(Story)
  upvoted = db.ListProperty(db.Key)
  flagged = db.ListProperty(db.Key)

So when you want to see if a user upvoted for a given story:

Tracking.all().filter('story =', db.Key.from_path('Story', uid)).filter('upvoted =', db.Key.from_path('UserProfile', uid)).get(keys_only=True)

Now the only problem here is the size of the upvoted/flagged lists can’t grow too large (I think the limit is 5000), so you’d have to make a class to manage this (that is, when adding to the upvoted/flagged lists, detect if X entries exists, and if so, start a new tracking object to hold additional values). You will also have to make this transactional and with HR you have a 1 write per second threshold. This may or may not be an issue depending on your expected use case. A way around the write threshold would be to implement upvotes/flags using pull-queues and to have a cron job that pulls and batch updates tracking objects as needed.

This method has its pros/cons. The most obvious cons are the ones I just listed. The pros, however, may be worth it. You can get a full list of users who upvoted/flagged a story from a single list (or multiple depending on how popular the story is). You can get a full list of users with a lot fewer queries to the datastore. This method should also take less storage, index, and metadata space. Additionally, adding a user to a tracking object will be cheaper, instead of writing a new object + 2 writes for each property, you would just be charged 1 write for the object + 2 writes for the entry to the list (9 vs 3 writes for adding users to a pre-existing tracked story, or 9 vs 7 for untracked stories)