Getting line length of geometry of parent feature as default value in child feature in QGIS

Question

I have:

• a parent line-layer 'MyParentLineLayer' (referenced layer) with a field "parent_uuid" (referenced field).
• a child geometryless-layer 'MyChildLayer' (referencing layer) with a field "parent_uuid_ref" (referencing field).

I set up a relation between parent layer and child-layer as described before in project properties.

In parent-layer-properties - myRelation the checkbox "Force hide form on add feature" is checked.

I would like to get the line length of the parent-feature as a (editable) default-value in the child-features-form.

I tried:

length(
    geometry(
        get_feature('MyParentLineLayer', 'parent_uuid', current_value('parent_uuid_ref'))
        )
    )

as default-value-expression (widget type: text-editor, field type: double) but always got a NULL-value as line-length.

What is wrong with my code/settings?

Answer 1

update: answer below is only relevant when adding new features in child layer. Question has since been clarified to the child form that appears when adding feature to parent layer, and using a default value that depends on unsaved parent form values. Disregard answer below.

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I believe current_value() is used more for widgets that are themselves configured to use a relation reference. Your line length widget in the child layer is just a simple widget with a default value so current_value() doesn't really work (that's my understanding).

Your expression should work if you use "parent_uuid_ref" instead of current_value('parent_uuid_ref').

i.e.,

length(
    geometry(
        get_feature('MyParentLineLayer','parent_uuid',"parent_uuid_ref")
        )
    )

See below for example using above expression in default value field.