Why not use a ReferenceProperty pointing to a User in the SS model instead of an unindexed StringProperty? The User model can then use the backreference collection to get a list of photos owned by the user.
I don't see a problem with a simple ListProperty of favorites, although making this a list of db.Keys instead of a list of integer IDs is probably preferable, because if you do decide to make photos children of another entity the IDs won't be globally unique while the db.Keys still will be. On Jan 23, 7:29 pm, dburns <drrnb...@gmail.com> wrote: > I'd appreciate any insight into the best design for this problem. I > have a photo-sharing app. I want people to be able to mark a photo as > a "favourite", just as YouTube does with videos. > > I have two kinds: Snapshots and Users. Given a user, I need to be > able to get a list of their favourites, and a list of the snapshots > they created. I need the inverse too, i.e. given a snapshot, I need > to know who made it. > > The problem I have is ensuring consistency across these two kinds, but > I don't think I can put them into the same entity group. First, here > they are: > > # The snapshot class. > class SS(db.Model): > owner = db.StringProperty(indexed=False) # Who created this (user > id). > #Other data about the snapshot here > > # The user class (the key_name is the user id) > class User(db.Model): > owned_ids = db.ListProperty(int, indexed=False) # IDs of owned > snapshots (i.e. created by this user) > fav_ids = db.ListProperty(int, indexed=False) # IDs of > favourite snapshots > > The issue is that when a user either creates or deletes a snapshot, > there's a potential for those to get out of sync if an exception > happens just at the wrong moment (e.g. a snapshot could exist where > the creating user doesn't have it in the owned_ids list). > > Making a User instance the parent of a SS (snapshot) instance seems > like a natural fit, except then I can't fetch all the favourites via: > favs = SS.get_by_id(user.fav_ids). The reason is that all parents > have to be the same to use SS.get_by_id (according > tohttp://code.google.com/appengine/docs/python/datastore/modelclass.htm...), > but those favourites may have been created by various users (hence the > parents would be different). > > Originally I had no owned_ids in User, and did a query to find that > user's snapshots (owner in SS was indexed). But that was slow and > didn't lend itself to paging. So I switched to get_by_id. > > Documentation note: run_in_transaction > athttp://code.google.com/appengine/docs/python/datastore/functions.html > doesn't mention the restriction that entities have to be in the same > group. I discovered it by seeing the exception, then read up in more > detail elsewhere. I naturally started there so it probably should be > mentioned. -- 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-appeng...@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.
