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 to
http://code.google.com/appengine/docs/python/datastore/modelclass.html#Model_get_by_id),
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 at
http://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.
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