Hi hawkett,
On Sat, Jun 20, 2009 at 3:05 PM, hawkett <hawk...@gmail.com> wrote:
>
> Hi,
>
> I was watching Brett's IO talk re. using 'Relational Index Tables',
> and there were a few hints of things in there, and I just wanted to
> check I got it all correctly -
>
> 1. Lists are good for tuples - a use case I see is an entity being
> tagged, and having a state within that tag - so the tuples might be
> ('tagA', 'PENDING') , ('tagB', 'ACCEPTED'), ('tagC', 'DENIED') etc. -
> so the list structures would be
>
> class Thing(db.Model):
> name = db.StringProperty()
> tags = db.ListProperty(str, default=[])
> states = db.ListProperty(str, default=[])
>
> with their contents tags = ['tagA', 'tagB', 'tagC'], states =
> ['PENDING', 'ACCEPTED', 'DENIED']
>
> and as data comes and goes you maintain both lists to ensure you
> record the correct state for the correct tag by matching their list
> position.
A much better approach is to use a single ListProperty, and serialize your
tuples to it - using Pickle, JSON, CSV, etc - whatever suits. If you want,
you can easily write a custom Datastore property class to make this easier.
This allows you to do everything you outlined below without extra effort.
-Nick Johnson
>
> 2. Relational Index Tables are good for exploding index problems - so
> the query here might be -
> "get me all the 'Things' which have 'tagA' and which are 'PENDING' in
> that tag" - i.e. all records with the tuple ('tagA, 'PENDING'), which
> would be a composite index over two list properties - an exploding
> index.
>
> So assuming I've got the above right, I'm trying to work out a few
> things
>
> a. Without relational index tables, what is the best way to construct
> the query - e.g.
>
> things = db.GqlQuery(
> "SELECT * FROM Thing "
> "WHERE tags = :1 AND states = :2", 'tagA', 'PENDING')
>
> which would get me anything that had 'tagA' at any point in the tags
> list, and anything that had a 'PENDING' at any point in the states
> list. This is potentially many more records than those that match the
> tuple. So then I have to do an in-memory cull of those records
> returned and work out which ones actually conform to the tuple? Just
> wondering if I am missing something here, because it seems like a
> great method for storing a tuple, but complex to query for that same
> tuple?
>
> b. If I am going to use relational index tables, to avoid the
> exploding index that the above query could generate -
>
> class Thing(db.model):
> name = db.StringProperty()
>
> class ThingTagIndex(db.Model)
> tags = db.ListProperty(str, default=[])
>
> class ThingStateIndex(db.Model)
> states = db.ListProperty(str, default=[])
>
> then am I right in thinking that my query would be performed as
>
> tagIndexKeys = db.GqlQuery(
> "SELECT __key__ FROM ThingTagIndex "
> "WHERE tags = :1", 'tagA')
>
> # All the things that have 'tagA' in their tags list
> thingTagKeys = [k.parent() for k in tagIndexKeys]
>
> stateIndexKeys = db.GqlQuery(
> "SELECT __key__ FROM ThingStateIndex "
> "WHERE states = :1 AND ANCESTOR IN :2", 'PENDING', thingTagKeys)
>
> # All the things that have both 'tagA' and 'PENDING' (but not
> necessarily as a tuple)
> thingKeys = [k.parent() for k in stateIndexKeys]
>
> things = db.get(thingKeys)
>
> # Oops - I need the lists to do the culling part of my tuple query
> from (a)
>
> So I have avoided the exploding index by performing two separate
> queries, but I could have achieved much the same result without the
> index tables - i.e. by performing separate queries and avoiding the
> composite index. Just wondering if I am seeing the tuple situation
> correctly - i.e. there is no way to query them that doesn't require
> some in-memory culling? Thanks,
>
> Colin
>
> >
>
--
Nick Johnson, App Engine Developer Programs Engineer
Google Ireland Ltd. :: Registered in Dublin, Ireland, Registration Number:
368047
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