Adding new field in point layer with attribute from another layer in QGIS

Question

I'm working with QGIS 3.16 Hannover under MacOS 10.13 environment. I've got a point and a polygon shapefile layers.

I want to add a new field to my point layer with the "id" of the polygon that contains it. Is it possible to do so with the field calculator, instead of creating a new layer?

Answer 1

You can use refFunctions Plugin. It adds custom user functions to QGIS Field Calculator. Then, you can add a new field to the point layer with the id of the polygon that contains it using the following expression in Field Calculator for the point layer:

geomwithin('polygon_layer_name', 'id_field_of_polygon_layer')

Answer 2

There is also a possibility without need to install a plugin, simply using this expression. overlay_within() is available since QGIS 3.16, in fact implementing the functions of the refFunctions plugin in QGIS natively: https://qgis.org/en/site/forusers/visualchangelog316/#port-reffunctions-to-core

array_to_string(overlay_within('polygon', id))

The first argument of the overlay_within() function is the layer it refers to, the second one an expression executed on this layer (here: simply a field name). Add the function to_int() to get an integer instead of a string as output type.

Answer 3

Currently a bit "old-fashioned" approach using the aggregate() function:

aggregate(layer:='polygon_layer',
          aggregate:='concatenate',
          concatenator:=',',
          expression:=to_string("id"),
          filter:=contains($geometry, geometry(@parent))
          )