XML模式：限制来自另一个名称空间的元素

Question

I'm writing an XML Schema for a document type that includes tags from other namespaces. (Atom and GML, to be specific: well-known standards.) Requiring a particular tag from another namespace seems easy enough:

<xs:import namespace="http://www.w3.org/2005/Atom" schemaLocation="atom.xsd" />
<!-- and then, within a complex type definition: -->
<xs:element ref="atom:link" />

But in several instances, I need to restrict the attributes on this tag. For example, I need to be able to specify the value of rel attribute of the atom:link tag. That is, I need to assert that a tag in the namespace I'm writing the schema for contain <atom:link rel="self" href="..." /> and not <atom:link rel="somethingelse" /> .

I know how to do this with an <xs:attribute /> tag on a complexType I'm defining myself, but I don't know how to do this on a tag which lives within another namespace. I can edit atom.xsd if necessary, but note that in different places in my schema I need to assert different rel values, so I can't just add a single rel restriction on every occurrence of atom:link .

Is this doable? Thanks!

Answer 1

Yes, it's a problem. To solve this problem XSD 1.1 allows targetNamespace to be specified on local element and attribute declarations (but only when you are refining a type in that namespace). If you don't have access to an XSD 1.1 processor, your only solution is to "hijack" the original namespace - define a schema document whose target namespace is the same as the namespace you are defining a restriction of, and put your restricted types in that namespace. Your element declarations, of course, can still go in your own namespace.