QGIS Expressions – How to Get Attribute Values of Another Layer

Question

The question: How is it possible to get the value of an attribute in another layer?

What is clear: When using QGIS expressions on a layer, it is possible to relate to features of another layer with get_feature(layer,attribute,value) (enter the value and get the first feature with this value) or get_feature_by_id(layer,feature_id) (you enter th id of the feature you want to get). Adding geometry(feature) gets the geometry.

The problem: I found no expression that allows to get the values of a certain field – like: enter the id of a feature and get the content of the field "value" – something like (pseudo-expression): attribute_value(feature). The idea is, by way of example, to have access to a point layer and get all the points where field "value" > 500.

This screenshot shows a successful connection (black line) from the start point of the red line to the point with the value = 1486. However, I want to get a connecting line from the start point of the line to all blue points with value > 500:

What I tried: I tested different combinations with aggregate(layer,aggregate,expression[,filter][,concatenator=''][,order_by]), however I was not able to get an aggregate of "value" in a numerical form, I always had to convert them to stings:

1. aggregate( 'points', aggregate:='concatenate', expression:="value", filter:= ("value" > 500), concatenator:=',') produces an error: Could not calculat aggregate for: value
2. The shorter version aggregate( 'points', 'concatenate', "value") produces an output of NULL
3. aggregate( 'points', 'concatenate', to_string ("value")) works, but produces a long string without delimeters, output looks like: 129594124217708841125620148663516710931369194910474691822123…' Adding concatenator:=',' does not help. But the main problem is that the values are converted to strings, so no way to use mathematical operators.
4. Update: the solution by MrXsquared helped me to create almost what I want, but if the values are not unique, it selects just the first feature. See this screenshot, I created the points with value > 500 as an additonal red points layer with geometry generator. There are two points with the value 513, but only one is created with the geometry generator , the other one not – in fact, the first one is rendered twice, as a duplicate. This is because get_feature () only gets the first feature that matches the input value.

This is the expression used:

collect_geometries (
    array_foreach (
        aggregate (
            'points',
            'array_agg',
            "value",
            "value">500 
        ) ,
    geometry (
        get_feature (
            'points', 
            'value', 
            @element
        )
    )
    )
)

I'm quite stuck here, what do I miss?

I want to get the feature id of all features with "value">500:

Answer 1

Instead of collecting the values and selecting back the geometry using this value, you can directly collect the geometries.

collect_geometries (
    array_foreach (
        aggregate (
            'points',
            'array_agg',
            $geometry,
            "value">500 
        ) ,
    make_line(start_point($geometry),@element)
    )
)