How to compare polygons in different layers in QGIS 3.x?

Question

To formulate my question with a concrete example: I have two multi-polygon layers of the same city.

• Layer A has polygons of the city divided in parts e.g. north, south, west, east.
• Layer B has smaller polygons that represent green e.g. trees, forests, parks and such.

Now I want to add a new column "city_part" in the attribute table of layer B. And likewise I'd want an either QGIS-expression- or Python-coded script, that can figure out in which polygon of layer A every single polygon of layer B lies. So the concrete result would be a layer B in which every polygon's got a new attribute, telling whether it resides inside north, south, west or east. How can this be done?

Answer 1

One way of many:

• install refFunctions plugin

• with your greens layer active

  • open Field Calculator

  • Create a new field

    • choose Output field name (e.g. 'city_part')

    • choose Output field type to match that of the city parts layers id column (ì.e. Text)

  • use expression:

    geomintersects( '<city_parts_layer_name>', '<city_parts_id_column_name>')
    

    and hit Ok

This assumes the city parts polygon id has the values north, south, east, west. It also assumes uniqueness of the intersections; if one of the smaller polygons intersects multiple city parts, it is internal order based which one will get assigned.