Referencing another layer in Field Calculator inside Graphical Modeler

Question

I want to use the Field Calculator inside the Graphical Modeler to create a new column in an input vector layer (LAYER1) using data in another layer (LAYER2). LAYER2 is the output of another algorithm inside the model.

The formula is as follows:

attribute(get_feature("LAYER2", 'WKT', geom_to_wkt(start_point($geometry))) , 'NODEID')

Actually, I am trying to implement an algorithm similar to what's described in Getting points-touching-a-line attributes back to lines layer using QGIS?.

When used outside the model, it works perfectly.

When used inside the model, the column is always created with NULL values.

Any ideas on how to reference LAYER2 inside the Field Calculator for this to work?

Answer 1

I found a possible workaround:

Seems that high level variables (e.g. input model parameters) can't be resolved inside nested expressions.

The trick is to do evaluation at higher level with some kind of lambda (e.g. string concatenation) then refer concatenation result in nested expressions.

I didn't investigate if the reason is that model scope is not propagated or not checked during expression evaluation.

Answer 2

The idea would be to create the join and then create the field:

1. Create geometry from WKT on LAYER2 using Geometry by expression
2. Extract the start point of LAYER2 using Extract specific vertices and specify first vertex (index 0).
3. Join using Join attributes by location using the output of previous alg and of LAYER1.
4. Use the Field Calculator using the joined field