Another idea may be to calculate the ratings as you go, since this is a somewhat expensive operation. For example, you could simply save 1 entity with 1 list property, [vote_sum,vote_total]
on submission, add their rating to vote_sum and increment vote_total, this way when you want an average you just divide entity.list_property[0] / entity.list_property[1] It obviously depends on your use case, but if you are calculating alot more averages than you are taking in new votes you'll want to make the calculation easier. Remember, this datastore is not relational, and averages are not as simple as a SQL AVG() function. On May 4, 11:31 pm, Robert Kluin <[email protected]> wrote: > Wow, I also forgot to include a few helpful links. > > The Datastore: > http://code.google.com/appengine/docs/python/datastore/ > > And, it sounds like you might want to explore some general Python resources > too: > http://python.org/doc/ > > Robert > > > > > > > > On Thu, May 5, 2011 at 00:23, Robert Kluin <[email protected]> wrote: > > Whoops, in the second method there is a typo. count() should be len(): > > average_rating = float(sum(ratings)) / len(ratings) > > > On Thu, May 5, 2011 at 00:23, Robert Kluin <[email protected]> wrote: > >> Hi Wilson, > >> You outlined the pattern to do this yourself. Fetch the entities > >> you want to average, add up the ratings, then divide by the number of > >> ratings. There are several ways to do this, in Python one very easy > >> to follow method might look like: > > >> # Get the ratings entities. > >> ratings_entities = Ratings.all().fetch(100) > > >> total_rating = 0.0 > >> ratings_count = 0 > >> for entity in ratings_entities: > >> total_rating += entity.rating > >> ratings_count += 1 > > >> average_rating = total_rating / ratings_count > > >> # Or, more concisely > >> ratings = [entity.rating for entity in ratings_entities] > >> average_rating = float(sum(ratings)) / count(ratings) > > >> However, note that you should *not* do this on every request. Your > >> app will perform terribly. Instead you should come up with a way to > >> compute and store the computed average rating. You could do this when > >> someone adds a rating, or periodically update the average. you'll > >> have to decide which method makes the most sense for you. > > >> Robert > > >> On Wed, May 4, 2011 at 02:52, Wilson Giese <[email protected]> wrote: > >>> I'm pretty new to databases and googles app engine, and I'm having a > >>> hard time getting saved data and averaging it. So basically we have a > >>> comments and ratings system where they can type comments and add > >>> ratings, but I can't figure out how to get all the ratings, and > >>> average them. I just simply do not know how to do it. > > >>> Is there any way I can pull saved data and average? > > >>> So say "rating" is in database, and there were 5 ratings currently. > >>> How would I get all ratings, add them together, and divide by the > >>> number of ratings. Is this even possible? > > >>> Sorry if this is confusing. > > >>> - Thanks in advance > > >>> -- > >>> You received this message because you are subscribed to the Google Groups > >>> "Google App Engine" group. > >>> To post to this group, send email to [email protected]. > >>> To unsubscribe from this group, send email to > >>> [email protected]. > >>> For more options, visit this group > >>> athttp://groups.google.com/group/google-appengine?hl=en. -- You received this message because you are subscribed to the Google Groups "Google App Engine" group. To post to this group, send email to [email protected]. To unsubscribe from this group, send email to [email protected]. For more options, visit this group at http://groups.google.com/group/google-appengine?hl=en.
