Using QGIS model output in same model as layer in field calculator formula

Question

I am trying to use field calculator within a model. In this formula I want to reference the output (temporary layer) of another step of the same model (output objektfilter_strassen_wege, which is named strassenpolygone_flurstuecke). Since this layer is not loaded in QGIS, I get no result. If I save the correct layer after running through the model and have the model repeated, I get a result. But that's not my goal! I want to get it the first time. I enter the following into the field calculator:

aggregate(
 layer:= 'strassenpolygone_flurstuecke',
 aggregate:='concatenate',
 expression:=oid,
 concatenator:=', ',
 filter:=intersects($geometry, geometry(@parent))
 )

strassenpolygone_flurstuecke should be called differently as input, I suspect. So not layer:= but something else that suggests that it is a generated output. The second possibility that went through my head was to choose to save the layer strassenpolygone_flurstuecke instead of the temporary layer. But even if I do this using the model, I don't get any result.

---

Answer 1

This is currently (QGIS 3.16.1) not possible, see the following comment from https://github.com/qgis/QGIS/issues/30397#issuecomment-505699624 :

The issue here is that expression functions which operate on map layers [...] don't currently have access to any temporary output layers created a step in a model. This applies to a lot of functions, including aggregates.

Here is another ticket describing your use case which is marked as a duplicate of the ticket linked above: https://github.com/qgis/QGIS/issues/37347

Answer 2

You definitely can reference a (temporary) output of an algorithm in a QGIS model in an expression. When you add an algorithm to the model, QGIS automatically create new variables for the output of that algorithm. You can find these variables listed wherever you have the expression string builder inside the model.

The variables are named like the algorithm with the suffix _OUTPUT_ and the leading @ that marks a variable, e.g. @Feature_filter_OUTPUT_:

Answer 3

I came across this post while looking for a solution to a similar problem. You just had to replace the layer name with @layer.

aggregate(
 layer:= @layer,
 aggregate:='concatenate',
 expression:=oid,
 concatenator:=', ',
 filter:=intersects($geometry, geometry(@parent))
 )