Hide fields from layout legend expr filter

[elpaso]

Fix #53244

[roya0045]

I don't think that hiding information that could be useful is a good fix. IMO the solution would be to have the preview operate on the symbols instead of the features. The fields can still be useful for aggregate operations.

[elpaso]

I don't think that hiding information that could be useful is a good fix. IMO the solution would be to have the preview operate on the symbols instead of the features. The fields can still be useful for aggregate operations.

I agree with you, a proper fix would require significant changes to the expression dialog. Unfortunately I don't think I have the time and resources for that implementation now but I would be glad to assist and review your code if you are willing to do it.

The question is if this small PR is a step forward or not.

If it isn't I'm happy to close it and wait for a better solution.

[roya0045]

Yes I came to the same conclusion that altering the expression builder would require a significant refactoring to solve this corner case. What could be helpful is to get #48375 merged, this would help point to user a robust formulation to perform aggregate calculations.

I don't know if that's possible, but displaying the field from the layer and having the feature be the atlas feature might give a more reliable preview. Though it might confuse users.

But as it stands, I'm afraid that simply hiding the fields will be a shortfix that will become permanent and will greatly impede the usage the of tool. I will try to see if I can come up with something to help shortly. Otherwise my opinion can be ignored if Nyall agree to move on with this quickfix for the moment.

The builder is at https://github.com/qgis/QGIS/blob/678cee7533ba9902a6e8483d8c35c96219a3e968/src/gui/layout/qgslayoutlegendwidget.cpp#LL1193C3-L1193C29

Maybe riging a way to use the layer's fields in

QGIS/src/gui/qgsexpressionbuilderwidget.cpp

Line 267 in 678cee7

void QgsExpressionBuilderWidget::initWithFields( const QgsFields &fields, const QgsExpressionContext &context, const QString &recentCollection, QgsExpressionBuilderWidget::Flags flags )

And then setting the layer to the atlas layer would make it work without too much refactoring.

[nyalldawson]

I'm happy with @elpaso's approach here -- the field names are still available for easy access from the 'Map Layers' group, it's just hiding the not-available "field" accessors here. That's a good approach for now.