Populate legend items with values from another layer

Question

I'm using QGIS 3.10.

I'm trying to label my map with values taken from a different layer (meaning not my atlas coverage layer), which is intersecting the atlas cover layer.

This layer named "Colorisé" has a categorized style using the field "color_id". I want my legend items to be labelled with the field "idprop".

I have tried using this expression (1), which returns the featured items not individually but as a list :

aggregate('Colorisé', 'concatenate_unique', "idprop", intersects($geometry,@atlas_geometry), ',', "idprop")

And I have tried using the expression below (2), which returns the first value it can find (I guess) and applies it to all my legend items :

attribute(get_feature('Colorisé', 'color_id', intersects($geometry, @atlas_geometry)), 'idprop')

Can you guys tell me if I'm anywhere close to getting the desired results ?

You can find below an excerpt from the table of attributes of the layer "Colorisé" and an image summing up the situation.

EDIT

Using this expression seems to be working in the preview, but the labels end up being empty :

attribute(get_feature(@layer,'color_id', 
attribute($currentfeature, 'color_id')),'idprop')

Answer 1

I thought about your problematic and may be your way is not the correct point of view, or mine isn't too.

What about adding the value "idprop" from your layer "colorised" into your atlas layer coverage? In the perspective of managing your database is more consistent : your don't multiply the source of data and so you can use it in different ways.

You didn't specify the quality of your layers, but i assumed there are polygons.

So you can use this algorithm "join attributes by location" or the next one 23.1.14.16. Join attributes by location (summary).

Answer 2

I actually got a working solution from another forum.

Here is the code :

aggregate(@layer, 'concatenate', "idprop", filter:="color_id"=@symbol_label AND intersects($geometry, @atlas_geometry), concatenator:=', ')

Hopefully this helps people who have had a similar issue as mine