Rule for labeling ways not being part of OSM relation in QGIS

Question

A brief overview of what I want to achieve: Create a map showing all cross-country ski trails in a specific region with some customizations/styling on them.

The data come from OpenStreetMap exclusively. Ski trails can be either relations or ways.

I imported the ways and relations from OpenStreetMap using QuickOSM plugin in QGIS 3 with the following Overpass query:

<osm-script output="xml" timeout="25">
    <union>
        <query type="way">
            <has-kv k="piste:type" v="nordic"/>
        </query>
        <query type="relation">
            <has-kv k="piste:type" v="nordic"/>
        </query>
    </union>
    <union>
        <item/>
        <recurse type="down"/>
    </union>
    <print mode="body"/>
</osm-script>

Now I have 2 layers in QGIS, one for lines (OSM ways) and one for multiline strings (OSM relations).

What I want to do is to add a label on ways (showing the value of the attribute piste:name or, if it doesn't exist, name) only if those ways are not part of a relation from the second layer. This is to avoid duplicating the label since these relations already have their name displayed with another labeling rule.

I tried to write rules using geometry functions like contains() but it seems that I can't reference an object from another layer.

Is there a way to achieve what I want to do in QGIS?

Answer 1

From askers comment:

I finally found a workaround to my issue. Instead of trying to test if a way is a member of a relation inside QGIS, I ran an additional Overpass API request to fetch ways that aren't par of any relation. Thus I now have a third layer with those ways, on which I can apply a simple labeling rule.

Answer 2

Plugin refFunctions will allow You to interact with geometry from another layer.

From the description:

Plugin that add custom user functions to Qgis Field calculator for referencing, analitically or spatially, between layers, For example retrieving a value from a layer using as a field value or a spatial condition (intersects, disjoint ....) as parameter

For example try geomcontains():

Retrieve target field value when source feature contains target feature in target layer