I'm trying to create a tool with graphical modeler of QGIS where I add a new attribute which represents length of the line divided by an certain number of interval informed in his attribute table. enter image description here

For that I use Field calculator algorithm where I write the expression (In pre-calculated value):

$length  / @AttributDivisionnumber

I tried to use:

$length / attribute(@AttributDivisionnumber) 

enter image description here enter image description here

But I have always the same error message No node roots. enter image description here I understand than Qgis consider than @AttributDivisionnumber is not a number when I use graphical modeler ... but idk how change it. Someone have any idea?

  • What does the error translate to in english?
    – Bera
    Commented Mar 14, 2023 at 18:30

1 Answer 1


You need to use eval() because the variable returns only the name of the field as a string, not the value(s) of this field:

$length / eval(@AttributDivisionnumber)

eval() Evaluates an expression which is passed in a string. Useful to expand dynamic parameters passed as context variables or fields.

  • Unfortunately It doesn't work, it keeps putting me 'No root node' ... Commented Mar 15, 2023 at 20:49

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