aggregates inside with_variable gives incorrect result

[puckipedia]

Describe the bug

When using any aggregate function (e.g. minimum) inside a with_variable, the result seems to be wrongly cached, even if it's not cachable.

How to Reproduce

• Create a layer with an id column, which can contain any value (as long as it's unique per row)
• Filter on "id" = with_variable('outer_id', "id", minimum("id", filter := "id" = @outer_id)) (guess who only just learned of @parent, lol)
• Notice that, while the filter expression should be a noop, the only result is the first row.

QGIS and OS versions

QGIS version	3.10.0-A Coruña	QGIS code branch	Release QGIS code branch.3
Compiled against Qt	5.12.5	Running against Qt	5.12.5
Compiled against GDAL/OGR	3.0.1	Running against GDAL/OGR	3.0.1
Compiled against GEOS	3.7.3-CAPI-1.11.3	Running against GEOS	3.7.3-CAPI-1.11.3 b50468f
Compiled against SQLite	3.30.0	Running against SQLite	3.30.0
PostgreSQL Client Version	11.5	SpatiaLite Version	4.3.0a
QWT Version	6.1.2	QScintilla2 Version	2.11.2
Compiled against PROJ	6.1.1	Running against PROJ	Rel. 6.1.1, July 1st, 2019
OS Version	NixOS 20.03pre-git (Markhor)
Active python plugins	pdokservicesplugin; db_manager; processing

[DelazJ]

Ah.... the combination would be the culprit...?
I was writing to the list last day because an expression i had, combining with_variable and aggregate, was outputting wrong result while I was pretty sure it was right. I'll then rewrite it the expanded way. Thanks.

(guess who only just learned of @parent, lol)

I had suggested to add it to the list of variables and not only inside examples to make it more visible but it did not get a positive reply, so we just add it in the user manual. I still think it's worth it.

[luipir]

I've the same problem and seems with_valiable is not expanded... I'm doing a query to an second layer, btw a simplified use case is updating a field in a layer getting a value from a second one basing a quey... e.g. in field calculator:

with_variable(
   'temp',
   "field name in first layer",
   aggregate(layer:='second layer name',
   	             aggregate:='min',
		     expression:="field name to get from second layer",
		     filter:="field name to check in the second layer"=@temp)
)

a simplified use case gives always wrong result and seems with_variable is not expanded (the same happen using eval)

with_variable(
	'temp',
	"field name in the first layer",
	aggregate(layer:='second layer name',
			aggregate:='max',
			expression:=@temp)
)

[luipir]

I'm using pre-release 3.16 master version 7216045

[3nids]

@luipir we were currently discussing the issue with @nirvn
a fix is on its way!

[luipir]

tnx @3nids building right now

[luipir]

@3nids in my case the PR fixes the issue.