Is it possible to create Rule-Based Styling with $geometry intersecting a second layer?

Question

I am trying to create a rule-based style based on geometries from another layer.

I have found an example that does exactly what I want, except that it uses a geometry from WKT:

intersects( $geometry, geomFromWKT( 'POLYGON' ) )

(from here)

What I want to do is exactly the same thing, but with the second geometry based on a different layer rather than WKT. As an example, I want to theme roads in black if they intersect a polygon layer, and grey if they do not. Is this possible?

Answer 1

In QGIS 2.18 there is a plugin available called refFunctions and when using the query builder, they show up in the Reference portion.

So this should work as a rule: geomintersects('polygon_layer','ogc_fid')

And possibly in conjunction with the geomdisjoint() function:

The field column in the above example just uses the ogc_fid primary key/id column but can be any column... Says it also accepts the $geometry variable but I haven't got that to work yet!

Answer 2

Unfortunately, it is currently not possible to create cross-layer queries using field calculator or expressions.

See also How to use the Intersects function in the Field Calculator of QGIS?

Answer 3

Yes, it is possible. In the 'rule based' symbology, choose the rule to be:

intersects($geometry, geometry(get_feature('layer name', 'attribute field name', 'value')))

Let's say you want your layer to intersect with geometry A (called 'A' in the field 'Name') in a second layer, called 'Other geometries'.

Then you would use:

intersects($geometry, geometry(get_feature('Other geometries', 'Name', 'A')))