QGIS Select features by expression: feature ids among attribute values from another layer using Model Builder

Question

I have two layers imported to QGIS 3.28.1 Firenze; 1* points and 1* polyline.

In Model Builder I have run an algorithm on the polyline which outputs points with an attribute named "KP"

Using the syntax

"KP" in (1,2,3,4)

I can select points from this layer where "KP" appears in a manually entered list. I am trying to select points from this layer where "KP" appears in the input points layer, attribute name "KPto".

I try

"KP" in (@KPzones_xyz, "KPto")

The script executes without error however nothing is returned. What should the correct syntax be?

QGIS Model Builder expression

enter image description here

• Erik: My magic 8-ball says you need to provide more details on your data, especially field names, contents and common keys.
• CJA888: Thanks. Field names of interest are "KPto" on layer KPZones_xyz and "KP" on layer "Field_calculator_OUTPUT". Both of which are type double. I don't understand the reference to magic 8-ball. I haven't come across this in QGIS before, is it a debugging tool?
• Erik: en.wikipedia.org/wiki/Magic_8_Ball - and I still have no idea, how your fields should interact. Is there a common key/ID? What exactly are you trying to achieve?
• CJA888: I see, thanks for the clarification on magic 8 ball, I have heard of it but didn't know what it does. I also have no idea how my fields should interact. No there is no common key. That is the problem, I am trying to select fields "KP" from my points layer where "KP" matches "KPto" from a different layer. You can think of "KPto" being a representation of (1,2,3,4) which was the manual text entry which works to select the required field.

Answer 1

If you run the select by expression on one layer to select the attributes pointid to be within all id fields from another layer layer1, one expression that will work is:

array_contains(
 aggregate(layer:='layer1', aggregate:='array_agg', expression:="id"),
 "pointid")

The aggregate() creates an array with all existing value of the layer1 attribute

array_contains() checks if the current layer attribute is part of the list

Applied to layer2, where attributes pointid are in the id of layer1:

enter image description here

---

If I get your attribute names well, it should read like this in your case:

array_contains(
 aggregate(layer:='KPzones_xyz', aggregate:='array_agg', expression:="KPto"),
 "KP")

• CJA888: Accepted as solution thank you- I removed explicit naming so code is as below for ease of reading, and kept the explicit case in comments array_contains( aggregate('KP_xyz', 'array_agg', "KPto"), "KP")