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I simply want to get a specific attribute (f.e. as label or default value in form) of a feature within another feature. Aggregate could do the job but it don't.

Hard for me to write it down, so let me explain on the screenshot below: All polygons are in the same layer. The green polygon is labeled with the expression:

aggregate('SameLayer','concatenate', to_string("Number"), intersects($geometry, geometry(@parent)))

That works fine so it tells me, I'm on the right way! All intersection features (101, 102, 103) are written to the label (or later to the attribute form). Since I just want the number of the the polygon, where the green one complete fits within, I changed 'intersects' to 'within' (or 'contains') and expect as result: 102 - but it don't!

aggregate('SameLayer','concatenate', to_string("Number"), within($geometry, geometry(@parent)))

The reason is, the green polygon touches with 3 of its sides, the parent polygon. If I draw the polygon complete within the parent, the expression works.

I wonder what could be the right geometry function in my case, and if Aggregate is the right way to solve that trivial task?

enter image description here

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2 Answers 2

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I don't know if something is off with how aggregate() handles geometric predicates (I am using QGIS 3.12.2)

I tried using the DE-9!M representation - according to relate() the relationship between the green polygon and 102 was '2FF11F212', so I used that to test the relationship instead of within(), but it kept returning an empty string (or NULL since the green polygon itself has a null value for "Number").

Eventually I got it to work with: aggregate('SameLayer','concatenate', to_string("Number"), relate($geometry,geometry(@parent),'212F11FF2'))

But, perhaps a crucial difference is that I also managed to get the expression to work with contains(), so maybe our geometries are not the same?? See if the above works for you...

enter image description here

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You can also use this expression that uses the overlay_within() function introduced in QGIS 3.16: array_to_string(overlay_within(@layer, "number"))

*Screenshot: I have 3 polygons (101,102,103) and a fourth on the same layer that lies within polygon 102 and has a NULL value for the attribute "number": I use this empty value for a rule based label with the expression from above:

enter image description here

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