[Qgis-user] Problem using Model Inputs variable in an Select by expression in the model Designer

Mon Aug 23 15:07:14 PDT 2021

I am a bit late, and Charles already provided a working solution, but don't
forget the extremely useful function eval.

The function as is will compare fieldname to fieldname (returning true) and
then try to concatenate the string '2' to it...

@NamelengthField != @NamelengthField || '2'

Try this:

@NamelengthField || '!=' || @NamelengthField || '2'

It will return the expression you need.

'length != length2'

 Now, just wrap it on eval() to calculate it.

eval(@NamelengthField || '!=' || @NamelengthField || '2')

Best regards

Alexandre Neto
User support
www.qcooperative.net

A segunda, 23/08/2021, 18:53, Charles Dixon-Paver <charles at kartoza.com>
escreveu:

> Honestly I have no idea 😂
>
> I mean, of course what is happening is QGIS is needing to identify a field
> from a text string, and it's even possible that with some convoluted chain
> of escape characters or funky regex the process could correct itself and
> "just work".
>
> In all likelihood you could have just stumbled onto some obscure bug in
> the underlying processing API where the context of that field object get's
> dropped or orphaned when  trying to access it.
>
> My assumption is that using attribute() just compensates for whatever is
> going on under the hood by reinstantiating the context behind the field
> object, or at the very least provides a robust interface for referencing it
> ¯\_(ツ)_/¯
>
> I would definitely call it a work around though
>
> On Mon, 23 Aug 2021 at 18:52, Nicolas Cadieux <njacadieux.gitlab at gmail.com>
> wrote:
>
>> Hi,
>>
>> if(attribute(to_string( @NamelengthField )) != "LENGTH2", TRUE, FALSE )
>>
>> ("LENGHT2" is an existing field and @NamelengthField was a model input
>> string.)
>>
>> This worked  for me!   If you have the time, please explain why I needed
>> the *attribute()* function for this to work.  I looked in the
>> documentation and did not find it very clear.
>>
>> Thanks a million you saved my butt!
>>
>> Nicolas
>>
>>
>> On 2021-07-20 5:53 a.m., Charles Dixon-Paver wrote:
>>
>> Hi Nicholas,
>>
>> I haven't had time to dig into this extensively or encountered this
>> particular issue, but I would suggest trying to "force" the expression to
>> retrieve the correct info regardless of context using something like this:
>>
>> with_variable('Length2',  to_string(@NamelengthField) || '2',
>> if(attribute(to_string(@NamelengthField)) != attribute(@Length2), TRUE,
>> FALSE))
>>
>> That's probably an extreme example but hopefully it illustrates my point.
>> If you want to share a sample dataset/ model I can try to have a look at
>> how to resolve it.
>>
>> Regards
>>
>> On Tue, 20 Jul 2021 at 10:46, Richard McDonnell <richard.mcdonnell at opw.ie>
>> wrote:
>>
>>> Nicolas,
>>>
>>> I have not looked at this in too much detail, but at a quick glance,
>>> could it not be the fact that you should be using the newly created fields?
>>> As in instead of *@NamelengthField != @NamelengthField || '2'*, should
>>> it not be more like your previous example *Length != Length2*
>>>
>>>
>>>
>>> Regards,
>>>
>>>
>>>
>>> Richard
>>>
>>>
>>>
>>>
>>>
>>> ——
>>> Richard McDonnell MSc GIS, FME Certified Professional
>>> *FRM Data Management*
>>>
>>> ——
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>>>
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>>>
>>> *From:* Qgis-user <qgis-user-bounces at lists.osgeo.org> *On Behalf Of *Nicolas
>>> Cadieux
>>> *Sent:* 20 July 2021 09:27
>>> *To:* Qgis Users List <qgis-user at lists.osgeo.org>
>>> *Subject:* Re: [Qgis-user] Problem using Model Inputs variable in an
>>> Select by expression in the model Designer
>>>
>>>
>>>
>>> Hi,
>>>
>>> Does anyone have an idea?
>>>
>>> Nicolas Cadieux
>>>
>>> https://gitlab.com/njacadieux
>>>
>>>
>>>
>>> Le 8 juill. 2021 à 01:17, Nicolas Cadieux <njacadieux.gitlab at gmail.com>
>>> a écrit :
>>>
>>> 
>>>
>>> Hi,
>>>
>>> I am creating a model that fixes Network files before they are used in
>>> the QGIS Network analysis (Shortest path) tools.
>>>
>>> This is my work flow:
>>>
>>>    1. Create a "Length" field (value) and calculate $length in the
>>>    field calculator
>>>    2. Split the Multipart lines into Singlepart lines
>>>    3. Create a "Length*2*" field (value) and calculate $length in the
>>>    field calculator
>>>    4. Select by expression Length != Length2.  This basically gives me
>>>    a selection of line that have been modified by the Multipart to Singlepart
>>>    algorithm. (I get 8 results)
>>>
>>> This works well
>>>
>>> The problem is when I create and Input String to let the user create a
>>> "Length" field.  This become a variable called @NamelengthField. My work
>>> flow become:
>>>
>>>    1. User input "Length" in the Model input. (@NamelengthField)
>>>    2. Create a "Length" field using  @NamelengthField (as
>>>    pre-calculated value) and calculate $length in the field calculator: This
>>>    is OK
>>>    3. Split the Multipart lines into Singlepart lines: This is OK
>>>    4. Create a "Length2" field using  @NamelengthField ||'2'  (as
>>>    pre-calculated value) and calculate $length in the field calculator: This
>>>    is OK this field (Length2) is created and calculated properly.
>>>    5. I get stuck  when I try Select by expression using
>>>    @NamelengthField != @NamelengthField || '2' (as pre-calculated value) (I
>>>    get all lines back) and the test does not work.
>>>
>>> I tried stuff like
>>>
>>> @NamelengthField != (@NamelengthField || '2')
>>>
>>> '@NamelengthField != @NamelengthField || '2''
>>>
>>> "@NamelengthField != @NamelengthField || '2'"
>>>
>>> "@NamelengthField"|| !=|| "@NamelengthField || '2'"
>>>
>>> Even simple @NamelengthField = 5.286365157529550 (which should extract
>>> exactly one value).
>>>
>>> Any ideas?
>>>
>>> Nicolas
>>>
>>> QGIS 3.18.2 on Windows 10
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>> --
>>>
>>> Nicolas Cadieux
>>>
>>> https://gitlab.com/njacadieux
>>>
>>> _______________________________________________
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>>>
>> --
>> Nicolas Cadieuxhttps://gitlab.com/njacadieux
>>
>> _______________________________________________
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