The entity element in doctrine-mapping.xsd requires an element id in front of the first element field.
This leads to invalid XML files, where a inherited class doesn't need an ID on its own, but inherits one from its parent.
In the examples in the documentation,
the code, too, has no id element in front of the first field elements.
A minimal example, showing the inheritance problem, too, is attached. The validation error occurs on the field element of the "Cat" entity.