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.
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
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