Here is some additional info about ListProperty vs HABTM.
The same schema adding Membership tables instead of ListProperties:
db.define_table("Match",
SQLField("ground", "string"),
SQLField("result", "string"))
db.define_table("Team",
SQLField("name", "string"))
db.define_table("Player",
SQLField("name", "string"))
db.define_table("TeamMembership",
SQLField("team_id", "integer"),
SQLField("player_id", "integer"))
db.define_table("MatchMembership",
SQLField("team_id", "integer"),
SQLField("match_id", "integer"))
This is mostly equivalent to using a ListProperty, however you need 1
query extra of a Membership table to get the 'list' of ids, but also
to add/delete N items in a Membership you need N writes.
Having a normalized (3NF) schema (with either ListProperty or HABTM)
without joins requires N+1 queries to fetch things like all the player
names on a team (which is poor performance and you only get ~30
queries/req). To get the performance back, you could selectively de-
normalize (store data redundantly), based on the queries that you
expect to perform. So if your app needs all the player names for a
given team or all team names for a given player without a join in 1
query, you could also store the names in the TeamMembership:
db.define_table("TeamMembership",
SQLField("team_id", "integer"),
SQLField("team_name"))
SQLField("player_id", "integer")
SQLField("player_name"))
In this example, N+1 queries is now 1 query to get the player names
from a given team or team names for a given player. The catch is that
if you need to change a name, you need to update the name for each
TeamMembership matching that id, which is N writes again...
The main benefit for ListProperty is that adding/removing N items from
the list is 1 write at your end, but bigtable still needs to build/
drop the N indexes and since its a synchronous call, it does not
return until it finishes, so using ListProperty in that one use case
should be cheaper than N writes. But, added to GAE in August, batch
writes across entity groups (
http://googleappengine.blogspot.com/2008/08/couple-datastore-updates.html
) so now you could add/remove N items from a membership in parallel
with one batched write from your end (web2py does not support this).
How much is the N add/remove performance difference between
ListProperty and Membership batch writes? That would be an
interesting benchmark to perform. Another interesting point is that
ListProperties are limited to ~5000 items.
Robin
On Jan 2, 9:49 am, "sudhakar m" <[email protected]> wrote:
> Yep got the source
> fromhttp://mdp.cti.depaul.edu/examples/static/1.55rc2/web2py_src.zip
>
> I will do some trials and post back.
>
> Sudhakar.M
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