Stepping through the code/trying this from the prompt would be instructive,
but yes, the database would get hit twice.
The first time (with something like result_count = qry.count()) hence,
using a native function which should be pretty well optimised and
sqlalchemy should 'do the right thing'.
And the second time, post the limit/offset, with the `for rw in qry` (Think
of the bit in-between as refining a sql string to be emitted)
So whilst yes, the database would get hit twice, the important thing to
note is that what's transported back to the browser is an int for the total
number of rows and then the subset (say, 25/30) rows.
Glad that helped!
Rob
Am Sonntag, 22. Juli 2012 01:38:14 UTC+1 schrieb Craig Small:
>
> On Sat, Jul 21, 2012 at 07:42:16AM -0700, Rob wrote:
> > Hi Craig,
> >
> > My turn to be a little confused....(!)
> > Simplest things first (and my very sincere apologies in advance if I
> tell
> > you anything you already know!!):
> > 1. Flexigrid / jqgrid -- two different libraries (functionally and
> > feature-wise, the latter is the more powerful)
> > 2. TW / TW2 -- two rather different 'engines'
> > 3. core.jqGridWidget -- perhaps think of this as access to the raw
> jqgrid
> > and a means of displaying rows of 'data'
> > 4. sqla.SQLAjqGridWidget -- a jqgrid which is closely bound to an
> > sqlalchemy object model.
> >
> > As you say, they all work but they (perhaps) serve different
> purposes. As
> > to flexigrid/jqgrid, your choice... but the latter is unquestionably
> more
> > advanced and I find it aesthetically more pleasing (as far as I'm
> aware
> > there's no TW2 widget/wrapper for flexigrid).
> >
> > As to TW / TW2 (and in fact as per the tutorial you linked to by
> Ralph
> > Bean), in a new project I simply switch TW off and use only the
> latter. As
> > per the docs etc. TW and TW2 can be used in the same project (ie they
> can
> > both be 'switched on' at the same time) but I prefer the syntax of
> the
> > latter and -- here I really hope that I'm not speaking out of line(!)
> --
> > consensus appears to be in favour of TW2, that's certainly where the
> > development is most active.
> >
> > Just very quickly, I'll address the question you asked about having
> to
> > query and "then throw away all but 25 of them". If you look at the
> > resulting jqgrid you'll see in the pager/navgrid "page 1 of P" and on
> the
> > right hand side, "records 1 to n of Z", where Z is the total number
> of
> > rows in the recordset. So, very simply let's say there are 10,000
> records
> > in your table, you've now filtered the recordset to show only those
> whose
> > first name is "Mary", and you're left with 300 records. If you look
> > carefully at the code I sent, this will now become immediately
> apparent
> > You query/filter --- and then count the total number records (the Z
> has to
> > come from somewhere, right? :) ) and then limit/offset depending on
> the
> > number of records to display per page (let's say 25) and the page
> number
> > you're on. (that make sense?!)
> >
> > So, time to close some doors! We're now talking about using jqgrid
> with
> > TW2: your choice of jqGridWidget vs SQLAjqGridWidget... And now it
> very
> > much depends on what it is that you're trying to accomplish and the
> > 'object model' you're using. Fundamentally, it's a question about
> 'layers
> > of abstraction':
> >
> > * the SQLAlchemy ORM sits on top of the 'expression language',
> > * SQLAjqGridWidget sits on top of (indeed, inherits from)
> jqGridWidget.
> > * (the jqgrid itself just wants to point at a URL which will give
> it a
> > json representation of the data)
> >
> > Very broadly speaking, if you have a simple object model and, all
> things
> > being equal, in order to query it you'd use the object model/ORM then
> the
> > SQLAjqGridWidget will almost certainly get you there... Essentially,
> the
> > SQLAjqGridWidget takes the job away from you and constructs the
> > [@expose('json')] controller method/query for you -- granted, you
> lose
> > visibility of what exactly is being queried but in a great many cases
> this
> > may well be exactly what you're looking for.
> >
> > If on the other hand the object model is more complex or you find
> yourself
> > needing (or wanting) to use the expression language directly (don't
> forget
> > that the ORM is an abstraction of the expression language, it's
> slower and
> > provides less fine-grained control) then you will almost certainly
> have to
> > provide the controller method (ie the code snippet that I provided
> > earlier) yourself. In a nutshell, if you want full control of the
> query
> > you'll be using jqGridWidget.
> >
> > I really hope that this explanation has revealed more than it's
> obscured,
> > but if you still have questions I'd be happy to try and help.
> It sure did clear things up Rob, thanks as its helped and I've got a
> clear path ahead now. A lot of my queries are not simple ones so that
> is sounding like your first suggestion is the way to go.
>
> This is probably a sqlalchemy question, but where does the SQLa hit the
> database? You see on line 4 you do your query, but line 8 you have limit
> and offset, between those lines its not pulling the entire set of rows
> in and then in line 8 throwing it away. I'm from the php mysql function
> days so is it just a matter of 'sqlalchemy does the right thing
> normally, dont worry about it'?
>
> --
> Craig Small VK2XLZ http://enc.com.au/ csmall at : enc.com.au
> Debian GNU/Linux http://www.debian.org/ csmall at : debian.org
> GPG fingerprint: 5D2F B320 B825 D939 04D2 0519 3938 F96B DF50 FEA5
>
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