Re: [xml-dev] Schema: defining occurences in choices?

[Jeni Tennison <jeni@jenitennison.com>]

Hi Martin,

> I have a question on defining occurences in choices within schema.
[snip]
> where testentry should be a base type which gets then restricted.
> The name element must have certain fixed values. Depending on some
> of these values I want to define occurences/restrictions (minOccurs,
> maxOccurs, etc.) for the whole element. How can I do that?

I'm not sure which thing you want to affect the occurrence of -- is it
the <content> elements? If so, this is known as a "co-occurrence
constraint" -- the value of an element is affecting the content model
that you want to use. I'm afraid that co-occurrence constraints aren't
well supported in W3C XML Schema. The closest that you can do is:

<?xml version="1.0" encoding="UTF-8"?>
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema";
           elementFormDefault="qualified"
           attributeFormDefault="unqualified">

<xs:element name="test">
  <xs:complexType>
    <xs:sequence>
      <xs:element name="testentry" maxOccurs="unbounded">
        <xs:complexType>
          <xs:complexContent>
            <xs:restriction base="testType">
              <xs:element name="name">
                <xs:simpleType>
                  <xs:restriction base="xs:string">
                    <xs:enumeration value="value1" />
                    <xs:enumeration value="value2" />
                  </xs:restriction>
                </xs:simpleType>
              </xs:element>
              <xs:element name="content" type="xs:string"
                          minOccurs="0" maxOccurs="unbounded" />
            </xs:restriction>
          </xs:complexContent>
        </xs:complexType>
      </xs:element>
    </xs:sequence>
  </xs:complexType>
</xs:element>

<xs:complexType name="testType">
  <xs:sequence>
    <xs:element name="name" type="xs:string"/>
    <xs:element name="content" type="xs:string"
                minOccurs="0" maxOccurs="unbounded" />
  </xs:sequence>
</xs:complexType>

</xs:schema>

You can then have an additional layer of constraints expressed using
Schematron rules. For example:

<sch:rule context="testentry[name = 'value1']">
  <sch:assert test="content">
    A testentry whose name element child is 'value1' must have at
    least one content element child.
  </sch:assert>
</sch:rule>

Or, of course, you could swap to RELAX NG, which supports this kind of
constraint very easily:

element test {
  element testentry {
    (element name { "value1" }, element content { text }+) |
    (element name { "value2" }, element content { text }*)
  }+
}

Cheers,

Jeni

---
Jeni Tennison
http://www.jenitennison.com/