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In QGIS 3.16.0 I have a postgis-polygon-layer with a field "array_polygon" which contains an array.

When digitizing a postgis-point-layer-point I would like to use the different array-values of the intersecting polygon in different fields as a default value to avoid lots of spatial queryies (one for every field).

The polygon layer has an "array_polygon"-field with type "QVariantList" and type name "_float8" and length "-1" and precision "0". The point layer has an "polygon_info"-fiel with type "QVariantList" and type name "_float8" and length "-1" and precision "0" too.

The "polygon_info"- field of the point-layer has a default value like

array_get(aggregate(
layer:= 'my_polygon_layer',
aggregate:='array_agg',
expression:=array_polygon,
filter:=intersects($geometry, geometry(@parent))),0)

The preview shows array-values as expected:

But the "polygon-info"-field in my point-layer stays empty after digitizing a point over a polygon: enter image description here

(I tried the expression without "array_get("...",0) too but without success).

My other fields - which should refer to "polygon_info"-array have a default value like

array_get( "polygon_info" ,0)

(I checked

array_get( current_value("polygon_info") ,0)

too) but every approach has a NULL-result too.

What is wrong?

  1. Has the expression or field-definition to be different to store the evaluated array (as seen in preview) in my point-attribute-table ?
  2. Is the spatial-query not carried out before the default-values are set - how can I force QGIS to make the spatial query first and after that extracting the values from this field?
  3. Or is my expression wrong - which alternative is correct ?
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6
  • Does array_to_string() on a string field work? If so, its clearly a datatype issue.
    – MrXsquared
    Jan 19, 2021 at 17:48
  • As you already use QGIS 3.16, you could try using the new overlay_intersects expression. Something like (pseudo expression) if ( overly_intersescts, your_expression, empty). And, as mentioned by @MrXsquared, try converting the output to a string and create a string field for testing
    – Babel
    Jan 19, 2021 at 22:42
  • also for testing, maybe get the first element in the array and write to string, or similar.
    – nr_aus
    Jan 20, 2021 at 2:54
  • @nr_aus: the expression already gets the first element in the array, see it's basic structure: array_get(array,0) - 0 for first element.
    – Babel
    Jan 20, 2021 at 10:57
  • 1
    @babel, thanks for that - I saw that after I posted, forgot to correct it....... you guys are saying what im thinking anyways.... maybe also try some of the other array_ methods... array_first also seems a bit more suitable in this instance. array_find method may also prove helpful with debug.
    – nr_aus
    Jan 20, 2021 at 14:05

1 Answer 1

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For debugging reasons I made a new fresh project with just this two layers directly from database by browser (and afterwards by inserting the point-layer as QLR-file too). Both point-layer-arrays now worked in the new project as aimed/expected.

In my old project I inserted the QLR from the "old" layer. But the "old" layer and the inserted layer from QLR still didn't work. Then I inserted the point layer via browser from postgresql-database and copied the code from the "old"-layer as default-value to the new point-layer. Surprisingly the attribute-field was filled now in the new layer of the "old" project.

So maybe there is a bug in my project/layer-definition (there are several spatial queries / default values / constraints / value relations in this layer and relations to other child-layers).

I'll try to narrow down the reason. But however - code and dataformat was not wrong. Maybe I can set up the layer newly with his settings without getting this problems again.

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