XSD key/keyref beginner question

Question

I\'m trying to implement a very simple XML schema constraint.

The idref attribute on elements of type should only be

Answer 1

Even with an assigned schema location this will not work in all parsers.

This will validate as well, because the key is not referencing the target namespace.

Changes that need to be made in the XSD are

And

For a discussion regarding this behaviour see #1545101