QGIS 3.14: default value

Question

I have a question about "default value" based on expression.

For exeaple, I have a layer in a GPKG, with 3 attributes: "Number" / "Letter" / "Roman".

For my attribute "Number", I have a value from a list with 1, 2 and 3. For the attribute "Letter", this attribute is not editable. In "default value", I have this expression:

IF("Number"=1, 'one', IF("Number"=2, 'two','three'))

This step work fine, and if I change the value "Number", the "Letter" is updated... Perfect!

Now, if I do the same for the attribute "Roman", but based on "Letter", calculated previously:

IF("Letter" like 'one', 'I', IF("Letter" like 'two', 'II','III'))

But it doesn't work. The attribute isn't calculated, and isn't updated when a change occurs.

Answer 1

In QGIS' documentation on form default values, it states:

The Default value option is not aware of the values in any other field of the feature being created so it won’t be possible to use an expression combining any of those values i.e using an expression like concat(field1, field2) may not work.

So in your example, you shouldn't expect "Letter" to have been set when the default value for "Roman" is being evaluated. You can't even expect the expression for "Letter" to know about "Number", though it seems in your case, the execution order happens to be such that it does.

In general, in instances where you think you want an attribute which is not editable but set on the basis of other attributes, it's better to use virtual fields. These are recalculated on the fly. This is rather than actual attribute fields with default values, since your user is not expected to change it anyway away from what the logic you have specified prescribes.