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<p>Hi,</p>
<p>I like the idea of a nicer API to combine several filter
conditions. Currently the only possibility to perform this
operation is to write a filter expression as a string or by
combining expression nodes.</p>
<p>I am not convinced however that this should be done directly on
QgsFilterRequest level. It feels a bit like reimplementing the
expression engine on another level again.</p>
<p>Some questions that come to my mind:</p>
<p> * Are these filters always combined with AND or also OR?</p>
<p> * Can these filters eventually be nested?<br>
</p>
<p> * How should we deal with compiling expressions, should that be
implemented again?</p>
<p>An alternative approach would be to work on a nicer API for an
expression builder to require 1) less string manipulation when
building an expression and 2) have more introspection capabilities
when manipulating an expression.</p>
<p>Regards</p>
<p>Matthias<br>
</p>
<div class="moz-cite-prefix">On 4/29/19 9:02 PM, Alexis R.L. wrote:<br>
</div>
<blockquote type="cite"
cite="mid:CAOM0XWC3hxoft-ToZY9fzmW1DKJANmLiR3GqoZhC6A8MxQdvFQ@mail.gmail.com">
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<div dir="ltr">Greetings Mr. Dawson,
<div><br>
</div>
<div>Thanks for the reply,</div>
<div><br>
</div>
<div>The tests that I did with the method in my PR was provider
agnostic but still entailed looping over every feature to do
the check. Could you provide idea or methods that could enable
this behaviour in an API friendly way?</div>
<div><br>
</div>
<div>Should I make a new iterator entirely or should I modify
existing methods and add a trigger like I tested in my PR?</div>
<div><br>
</div>
<div>Thanks!<br clear="all">
<div>
<div dir="ltr" class="gmail_signature"
data-smartmail="gmail_signature">
<div dir="ltr">Alexis Roy-Lizotte</div>
</div>
</div>
<br>
</div>
</div>
<br>
<div class="gmail_quote">
<div dir="ltr" class="gmail_attr">Le dim. 28 avr. 2019 à 19:46,
Nyall Dawson <<a href="mailto:nyall.dawson@gmail.com"
moz-do-not-send="true">nyall.dawson@gmail.com</a>> a
écrit :<br>
</div>
<blockquote class="gmail_quote" style="margin:0px 0px 0px
0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex">On
Sun, 28 Apr 2019 at 07:18, Alexis R.L. <<a
href="mailto:alroyliz0@gmail.com" target="_blank"
moz-do-not-send="true">alroyliz0@gmail.com</a>> wrote:<br>
><br>
> Greetings Everyone,<br>
><br>
> While working on my PR (9648) I noticed that expression
filter and a list of feature IDs are both considered the same
thing cannot be used as two filtering criteria.<br>
><br>
> I am aware that the intent was to only have one element
active to filter out features other than the rectangle(? might
be wrong on that one).<br>
><br>
> What I am wondering if it would be a good thing to have
both co exist (mostly for my PR as of now). A simply way to do
so would be to use the give feature ids as the list to iterate
over and check for the expression if there is one.<br>
<br>
I also think this would be desirable. The tricky part (well,
one<br>
tricky part -- feature iterators are low level stuff, lots of<br>
complexity there!) is avoiding any API breakage while adding
new<br>
methods to handle your use case.<br>
<br>
The other (SUPER) tricky bit is that any change to feature
requests<br>
usually requires accompanying changes to every vector data
provider in<br>
order to keep consistent behaviour across all these providers.
The<br>
same request should always give the same result, regardless of
the<br>
actual underlying data provider. The consequence of this is
that<br>
you'll need to be update the postgres, mssql, oracle, OGR,
memory,<br>
spatialite, wfs, arcgis feature server, db2, and vector layer
feature<br>
iterator code. You'll also need to add many new unit tests to
cover<br>
all the changes to the different providers.<br>
<br>
Definitely not a trivial change! Don't let this put you off
making it,<br>
but just be aware that this change would take even a QGIS core<br>
developer a week or more of work to implement.<br>
<br>
Nyall<br>
<br>
<br>
<br>
><br>
> I assume that such a thing might be implemented without
touching the current behaviour, otherwise one would have to
remove either the expression or the IDs in some case when we
want to override the current filtering method.<br>
><br>
> Using multiple filtering method might be better than only
forcing one and would provide more flexibility.<br>
><br>
> As I have a PR that is related to this (<a
href="https://github.com/qgis/QGIS/pull/9816"
rel="noreferrer" target="_blank" moz-do-not-send="true">https://github.com/qgis/QGIS/pull/9816</a>
) I would like to address this point in it and would like to
have the discussion occur there as to the best way to
implement such a thing without any breakage if people are not
opposed to having only one filtering method in
qgsfeaturerequest.<br>
><br>
> Thanks and have a nice day!<br>
><br>
> Alex<br>
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