Error using QGIS expression aggregate

Question

I several times had the same problem using the QGIS expression aggregate: I got the impression that it behaves somehow buggy - or I make a mistake. See this expression I use on a point layer:

aggregate ( 
    layer:='points', 
    aggregate:='concatenate_unique',  
    expression:=x($geometry),
    concatenator:=','
)

The preview shows an error: Could not calculate aggregate for: x($geometry). If I click the arrow to the right of the preview to change to the preview for the next feature, the preview changes from error to NULL. As far as I can see, comparing with the help on the right side, the syntax is correct. Simplifying the expression to aggregate ('points','concatenate_unique', x($geometry)) results in exactely the same problem. I also changed the layer name to the @layer variable and used an existing field id to aggregate: aggregate (@layer,'concatenate_unique', "id" ). However, the very same problem persists.

Is this a bug or do I make a mistake?

Answer 1

You need to convert your coordinates to a string first:

aggregate ( 
    layer:='points', 
    aggregate:='concatenate_unique',  
    expression:=to_string(x($geometry)),
    concatenator:=','
)

This is because double values cannot be concatenated as a string and therefore returning NULL or Could not calculate aggregate for: $geometry. So it is related to 'concatenate' as well as 'concatenate_unique'. Therefore an alternative would be to use 'array_agg' instead:

array_to_string( -- concatenate the array to a comma separated string, as specified in the last line of this expression
    array_distinct( -- keep only unique values in this array
        aggregate(
            layer:='points',
            aggregate:='array_agg', -- create the array
            expression:=x($geometry)
        )
    )
,',') -- use comma as separator for concatenation