On Tue, Jun 23, 2009 at 12:42 PM, herbie <4whi...@o2.co.uk> wrote:
>
>
> So will this :
> query = Foo.all().filter("property_x >" 50).order("property_x") .order
> ("-timestamp")
> results = query.fetch(200)
>
> ..get the latest entities where property_x > 50 ? Or will it get the
> 200 properties with the largest 'property_x' which are then ordered
> by 'timestamp' ? A subtle but important difference.
It will get the 200 entities with the smallest property_x greater than 50
(since you're filtering >50 and ordering first by property_x). If two
entities have the same value for property_x, they will be sorted by
timestamp, descending.
If you need the latest, and your threshold of 50 is a constant, you can add
a BooleanProperty to your entity group encoding the condition 'is greater
than 50', and filter on that using an equality filter.
-Nick Johnson
>
> As I said I need make sure I get the latest entities.
>
>
> On Jun 22, 11:33 pm, Tony <fatd...@gmail.com> wrote:
> > Yes, that is what it means. I forgot about that restriction.
> >
> > I see what you mean about changing 'x' values. Perhaps consider
> > keeping two counts - a running sum and a running count (of the # of x
> > properties). If a user modifies an 'x' value, you can adjust the sum
> > up or down accordingly.
> >
> > On Jun 22, 5:40 pm, herbie <4whi...@o2.co.uk> wrote:
> >
> > > I tried your query below but I get "BadArgumentError: First ordering
> > > property must be the same as inequality filter property, if specified
> > > for this query;"
> > > Does this mean I have to order on 'x' first, then order on 'date'?
> > > Will this still return the latest 200 of all entities with x > 50 if
> > > I call query.fetch(200)?
> >
> > > I take your's and Nick's about keeping a 'running average'. But in
> > > my example the user can change the 'x' value so the average has to be
> > > recalculated from the latest entities.
> >
> > > On Jun 22, 9:46 pm, Tony <fatd...@gmail.com> wrote:
> >
> > > > You could accomplish this task like so:
> >
> > > > xlist = []
> > > > query = Foo.all().filter("property_x >" 50).order("-timestamp")
> > > > for q in query:
> > > > xlist.append(q.property_x)
> > > > avg = sum(xlist) / len(xlist)
> >
> > > > What Nick is saying, I think, is that fetching 1000 entities is going
> > > > to be very resource-intensive, so a better way to do it is to
> > > > calculate this data at write-time instead of read-time. For example,
> > > > every time you add an entity, you could update a separate entity that
> > > > has a property like "average = db.FloatProperty()" with the current
> > > > average, and then you could simply fetch that entity and get the
> > > > current running average.
> >
> > > > On Jun 22, 4:25 pm, herbie <4whi...@o2.co.uk> wrote:
> >
> > > > > Ok. Say I have many (>1000) Model entities with two properties 'x'
> > > > > and 'date'. What is the most efficient query to fetch say the
> > > > > latest 200 entities where x > 50. I don't care what their
> 'date's
> > > > > are as long as I get the latest and x > 50
> >
> > > > > Thanks again for your help.
> >
> > > > > On Jun 22, 4:11 pm, "Nick Johnson (Google)" <
> nick.john...@google.com>
> > > > > wrote:
> >
> > > > > > Consider precalculating this data and storing it against another
> entity.
> > > > > > This will save a lot of work on requests.
> >
> > > > > > -Nick Johnson
> >
> > > > > > On Mon, Jun 22, 2009 at 3:55 PM, herbie <4whi...@o2.co.uk>
> wrote:
> >
> > > > > > > No the users won't need to read 1000 entities, but I want to
> calculate
> > > > > > > the average of a property from the latest 1000 entities.
> >
> > > > > > > On Jun 22, 3:30 pm, "Nick Johnson (Google)" <
> nick.john...@google.com>
> > > > > > > wrote:
> > > > > > > > Correct. Are you sure you need 1000 entities, though? Your
> users probably
> > > > > > > > won't read through all 1000.
> >
> > > > > > > > -Nick Johnson
> >
> > > > > > > > On Mon, Jun 22, 2009 at 3:23 PM, herbie <4whi...@o2.co.uk>
> wrote:
> >
> > > > > > > > > So to be sure to get the latest 1000 entities I should add
> a datetime
> > > > > > > > > property to my entitie model and filter and sort on that?
> >
> > > > > > > > > On Jun 22, 1:42 pm, herbie <4whi...@o2.co.uk> wrote:
> > > > > > > > > > I know that if there are more than 1000 entities that
> match a query,
> > > > > > > > > > then only 1000 will be return by fetch(). But my
> question is which
> > > > > > > > > > 1000? The last 1000 added to the datastore? The first
> 1000 added to
> > > > > > > > > > the datastore? Or is it undedined?
> >
> > > > > > > > > > Thanks
> > > > > > > > > > Ian
> >
> > > > > > > > --
> > > > > > > > Nick Johnson, App Engine Developer Programs Engineer
> > > > > > > > Google Ireland Ltd. :: Registered in Dublin, Ireland,
> Registration
> > > > > > > Number:
> > > > > > > > 368047
> >
> > > > > > --
> > > > > > Nick Johnson, App Engine Developer Programs Engineer
> > > > > > Google Ireland Ltd. :: Registered in Dublin, Ireland,
> Registration Number:
> > > > > > 368047
> >
>
--
Nick Johnson, App Engine Developer Programs Engineer
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
-~----------~----~----~----~------~----~------~--~---
