How can QGIS Field Calculator sum values from a spatially joined table?

Question

Can the Field Calculater be used to sum values from a related table via a Spatial Join?

I have two vector polygon layers A and B, each with a numerical column VALUES. I want to update VALUES in layer A with the sum of VALUES from layer B where geometry from B is contained within geometry A.

So far I have the expression geomcontains('Layer B','VALUES') and what I want is to Sum this.

I have seen other ways to do this via plugins like SpatialJoin etc, but they all involve modifying either saving new layers or modifying the structure of Layer A, which I think is fairly cumbersome - is there a simple solution? (In MapInfo this is extremely simple, using Update Column.)

Answer 1

You can use the toolbox for the spatial join (processing toolbox > search: join by location) and save the result to a temporary layer. The temporary layer is only saved in your QGIS project. Then you can join the temporary layer to the layer A.

You can find the processing toolbox in view > panels > toolbox or right click over the toolbars.

Answer 2

The refFunctions plugin offers intersecting_geom_sum() function to do that task.

Activate the Field Calculator on your Layer A and try,

intersecting_geom_sum('Layer B','VALUES')

This function sums the geometries' attributes (Values in your case) of the target layer (your Layer B).