Using XPath can ID/IDREF use be checked?

Question

With XML Schema we can define ID/IDREF links between components in an XML document. I'm looking for a way, using XPath, to ensure that only certain IDs can be used as IDREFs - i.e. only those values which logically make senses in the context of the schema design.

Using the example below, how do I ensure that a book's library_id is actually one of the available library_ids and not some other id which could appear in the message and be used for some entirely different purpose (but still be schema valid).

<books>
    <book library_id="lib1">
        <author>A</author>
        <title>B</title>
    </book>
    <book library_id="lib2">
        <author>C</author>
        <title>D</title>
    </book>
    <library id="lib1">
        <name>X</name>
        <city>Y</city>
    </library>
    <library id="lib2">
        <name>X</name>
        <city>Y</city>
    </library>
</books>

The closest I've come to getting this to work is to use the XPath contains function e.g.

contains(//library/@id,//book/@library_id)

but this throws an error because a sequence of more than one item is not allowed as the first argument of contains() ("lib1","lib2").

Thanks

Answer 1

I hope that understood correctly what you want

Books with library_id present in library list

//book[@library_id = //library/@id]

Books with incorrect library_id

//book[not(@library_id = //library/@id)]