[QGIS-Developer] Feature's secondary geometry columns in expressions and python

[Olivier Dalang]

Dear list,

It's not exactly clear how secondary geometry columns are to be used from
QgsExpressions or Python code.

Just to be clear, by secondary geometry columns, I mean geometries stored
in other columns that the one that is used to display the features on the
map, typically a "center_point" field storing a Point on a Polygon layer.

When accessing them both in expression or Python, it seems it returns a
string with the ewkt representation (at least for postgis layers). This is
not really nice, as the only way to use it with functions such as make_line
is to strip the SRID from the text to get proper wkt, then to make the
create the feature using from_wkt.

This means instead of the nice and clear :
make_line( closest_point( $geometry, "center_point"), "center_point" )
You need :
make_line( closest_point( $geometry, geom_from_wkt( substr( "center_point",
strpos("center_point", ';') + 1) )), geom_from_wkt( substr( "center_point",
strpos("center_point", ';') + 1) ) )

It's also probably quite inefficient, which is an issue when used for
geometry generators.

Is there a limitation preventing to return QgsGeometries directly ? If not,
would it make sense to change QGIS to do so ? Does anyone have an efficient
workaround in the mean time ?

Thanks :-)

Olivier
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