QGIS Expression – Why Doesn’t with_variable Work in This QGIS Expression?

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I'm trying to make a filter inside an overlay_nearest function where I must relate an attribute from the current layer to the overlay layer.

I'm trying to use with_variable to store the outer layer attribute, but for a weird reason it's not working. Can someone shed some light on how to make it work?

This is the expression I'm using (where "ID" is an integer):

with_variable(
    'myId', 
    "ID", 
    overlay_nearest(
         'Lines', 
          $geometry, 
          filter:="ID"=@myId
    )
)

The expression above returns an array with 0 elements, but any of the expressions below work ok:

This shows that with_variable seems to be working correctly as it returns the expected ID -> 80000.

with_variable('myId', "ID", @myId)

This shows that using the filter with a correct ID also works, as it returns 1 element in the array

with_variable(
    'myId', 
    "ID", 
     overlay_nearest(
         'Lines', 
          $geometry, 
          filter:="ID"=80000
     )
)

So, if @myId has the correct value (80000), and filtering with "ID"=80000 works as expected, why does filtering with "ID"=@myId not work?

Best Answer

See my answer at: https://gis.stackexchange.com/a/477802/129409

Basically, it's a limitation in QGIS. There is a feature request open (https://github.com/qgis/QGIS/issues/43146), so it may become available at some point in the future.

In the meantime, the workaround is to wrap the overlay_nearest function in an eval call.

So rewriting your expression, you get:

with_variable('myId',"ID",
    eval( 'overlay_nearest( \'Lines\', $geometry, filter:="ID"=' || @myId || ' )' )
)
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