Select XML attributes on the root element

The app-schema plugin doesn't allow the selection of attributes on the root element; i.e. the selection of client properties of the target element. For example the following x-path on a fictious schema will work:

ns:root_element/ns:nested_element/@ns:attribute

but the following x-path will not work:

ns:root_element/@ns:attribute

This x-path selection mechanism is not disallowed by the OGC Filter Encoding standard.

I have a use-case where a filter is required to be applied to the attribute of the root element; currently this is not supported by the app-schema plugin.

I have provided a patch which includes test cases that extends FeatureTypeMapping and XPath to allow my use-case to proceed. mvn clean install on trunk passes all tests.

In implementing XPath.rootElementSteps() I have tried to follow XPath.steps() as a template. FeatureTypeMapping.findMappingsFor() has been altered, as little as possible, to take advantage of XPath.rootElementSteps() where appropraite.

Does the app-schema documentation need to be amended? IMHO I think it doesn't. This is just a correction to a deficiency, overlooked by the original designers because they didn't have a specific use-case.

I'm happy to answer questions or provided additional details about my use-case.