XML schema still allowing duplicate id's with unique

Question

I am trying to design an XML schema for books where a unique ID needs to be specified for each book entry. However it just doesn't seem to work. Below is the XSD i am using,

<?xml version="1.0"?>

<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema"
           elementFormDefault="qualified"
           attributeFormDefault="unqualified">

  <xs:element name="BookShelf">
    <xs:complexType>
      <xs:sequence>
        <xs:element name="Description" type="xs:string" minOccurs="0"/>
        <xs:element name="Shelf" type="ShelfType" minOccurs="1" maxOccurs="10"/>
      </xs:sequence>
    </xs:complexType>
  </xs:element>

  <xs:complexType name="ShelfType">
    <xs:sequence>
      <xs:element ref="Book" minOccurs="0" maxOccurs="unbounded"/>
    </xs:sequence>
  </xs:complexType>


<xs:element name="Book">
  <xs:complexType>
    <xs:sequence>
      <xs:element name="Title" type="xs:token"/>
      <xs:element name="Language" type="xs:language"/>
    </xs:sequence>
    <xs:attribute name="id" type="xs:string" use="required"/>
  </xs:complexType>
  <xs:unique name="unique-bookId">
    <xs:selector xpath="Book"/>
    <xs:field xpath="@id"/>
  </xs:unique>
</xs:element>
</xs:schema>

The XML I am trying to validate with this is,

<?xml version="1.0"?>

<BookShelf>
    <Description>My bookshelf</Description>
    <Shelf>
        <Book id="1">
            <Title>Seitsemän veljestä</Title>
            <Language>fi</Language>
        </Book>
        <Book id="1">
            <Title>Another title</Title>
            <Language>en</Language>
        </Book>
    </Shelf>
</BookShelf>

which is validating fine even though it should not (I've used the same id for 2 entries). I'm pretty new at XML and would appreciate if someone could please point out what I am doing wrong here?

Answer 1

You've got the <xs:unique> in the wrong place - it needs to be inside the definition of the ancestor element within which the Book elements should be unique, not in the Book element definition itself. The following would force Book ids to be unique within each shelf, but would allow the same ID on different shelves:

  <xs:element name="BookShelf">
    <xs:complexType>
      <xs:sequence>
        <xs:element name="Description" type="xs:string" minOccurs="0"/>
        <xs:element name="Shelf" type="ShelfType" minOccurs="1" maxOccurs="10">
          <xs:unique name="unique-bookId">
            <xs:selector xpath="Book"/><!-- selects books on this shelf -->
            <xs:field xpath="@id"/>
          </xs:unique>
        </xs:element>
      </xs:sequence>
    </xs:complexType>
  </xs:element>

If instead you want the IDs to be globally unique across all shelves then put the unique constraint at the BookShelf level and adjust the selector appropriately:

  <xs:element name="BookShelf">
    <xs:complexType>
      <xs:sequence>
        <xs:element name="Description" type="xs:string" minOccurs="0"/>
        <xs:element name="Shelf" type="ShelfType" minOccurs="1" maxOccurs="10"/>
      </xs:sequence>
    </xs:complexType>
    <xs:unique name="unique-bookId">
      <xs:selector xpath="Shelf/Book"/><!-- selects books on all shelves -->
      <xs:field xpath="@id"/>
    </xs:unique>
  </xs:element>

For future reference, note that if your schema had a targetNamespace then those selectors would not work as-is, because unprefixed names in selector XPaths always mean "no namespace". You would need to add xmlns:tns="<target namespace URI>" to your xs:schema element and then use a selector of tns:Shelf/tns:Book.

Answer 2

Do you specifically need to use digits as ID values? If not, one possibility is using xs:ID as the id attribute type instead of xs:string:

<xs:attribute name="id" type="xs:ID" use="required"/>

That way only unique XML identifiers are allowed as values of the id attribute. A valid XML identifier can't start with a digit, however, so you'll have to change your ID type to something like id-1, for example.