Filling in missing attribute data based on other features using QGIS expression

Question

I have a survey form and some features (trees) will be grouped using the same "TreeNumber".

Hopefully, I can write an expression that means users only have to fill in most of the data for one feature, and the expression with fill in the gaps in another set of virtual fields for the other features.

So far I have tried the expression below which I hoped would return the maximum value from all the features filtered to match the current feature "TreeNumber" but it just crashes QGIS.

maximum("Species", "TreeNumber" = attribute($currentfeature, "TreeNumber"))

Answer 1

Try this expression:

with_variable(
    name:='currentTreeNumber',
    value:="TreeNumber",
    expression:=maximum(
        expression:="Species",
        filter:="TreeNumber"=@currentTreeNumber
        )
    )

$currentfeature inside an aggregate returns features of the aggregation, you can store the value of the field of interest of the current feature in a variable with the with_variable() function and then compare it with the value inside the aggregate.