Hi Shailen, On Sun, Oct 11, 2009 at 4:09 PM, Shailen <shailen.t...@gmail.com> wrote:
> > Consider 2 classes: > > class Light(db.Model): > location = db.StringProperty() > > class Heavy(db.Model): > location = db.StringProperty() > content = db.TextProperty() > image1 = db.BlobProperty() > image2 = db.BLobProperty() > image3 = db.BlobProperty() > ..., etc., etc. ... > > I have 2 choices when getting information from the datastore: I can > either fetch just the keys, or I can > fetch the entities themselves. There isn't any way to fetch only a > part of an entity (say just the location property in entities of type > Heavy). Assuming this is correct, I have a couple of questions: > > 1) If I want to access the location property in both Light and Heavy, > is it fair to assume that accessing Light will be speedier than > accessing Heavy, since Heavy has to fetch all sorts of data that I > don't need but that is part of the entity? > Yes. > > 2) Would it make sense to break down Heavy into more lightweight > classes for more efficient lookup? > In the example above, definitely, yes. > > This question is probably best clarified by discussing what's > happening in my actual application where I allow an Author to create > several WriteUps. The WriteUps have numerous properties: a heading, a > sub-heading, a lot of text and, optionally, several images, and a few > more properties. When displaying the contents of a WriteUp, I need to > access all these properties. But sometimes, I need to access only 1 or > 2 lightweight properties; for instance, on a page summarizing the > WriteUps done by an author, I need to only list the heading and sub- > heading; I do not need to access the text or Blob data. But there is > no way to access *just* the heading or sub-heading, right? I end up > fetching ALL of the entity, whether I need all of it or not. Is this > correct? Or does the datastore have some way of optimizing this? > Entities are always fetched in their entirety. > > Reading the In the documentation (in the Queries and Indexes section), > we are told: > > # The query is not executed until results are accessed. > results = q.fetch(5) > for p in results: > print "%s %s, %d inches tall" % (p.first_name, p.last_name, > p.height) > > When are results 'accessed', when fetch() is called, or when > p.first_name, p._last_name, etc. are used? I am assuming that when > fetch(5) is called, all 5 entities (assuming they exist) are fetched, > and if the entities contain properties other than first_name, > last_name or height, they are loaded up too. Is that correct? > Yes. > > So now I am thinking of breaking up the WriteUps class into several > classes, putting all the lightweight data that I need to access > frequently in one class and all the heavy data that I rarely need to > access in another and using ancestor relationships and transactions to > make sure everything stays together. > Does this make any sense, or I am fundamentally misunderstanding how > app engine fetches data? > Yes, that sounds like a good idea. -Nick Johnson > > - Shailen > > > > > > > > > -- Nick Johnson, Developer Programs Engineer, App Engine Google Ireland Ltd. :: Registered in Dublin, Ireland, Registration Number: 368047 --~--~---------~--~----~------------~-------~--~----~ You received this message because you are subscribed to the Google Groups "Google App Engine" group. To post to this group, send email to google-appengine@googlegroups.com To unsubscribe from this group, send email to google-appengine+unsubscr...@googlegroups.com For more options, visit this group at http://groups.google.com/group/google-appengine?hl=en -~----------~----~----~----~------~----~------~--~---
