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

[Nicolas Cadieux]

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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