XSD restrict content of element depending on attributes value

Question

Formally our XSD looked like this:

<?xml version="1.0" encoding="utf-8"?>
<xs:schema id="fooxsd" xmlns:xs="http://www.w3.org/2001/XMLSchema" elementFormDefault="qualified">

  <xs:element name="List">
    <xs:complexType>
      <xs:sequence>
        <xs:element name="Config" type="ConfigType" minOccurs="0" maxOccurs="unbounded"/>
      </xs:sequence>
    </xs:complexType>
  </xs:element>

  <xs:complexType name="Entry">
    <xs:simpleContent>
      <xs:extension base="xs:string">
        <xs:attribute name="key" type="xs:ID" use="required" />
      </xs:extension>
    </xs:simpleContent>
  </xs:complexType>

</xs:schema>

XMLs can look like this:

<?xml version="1.0" encoding="utf-8"?>
<List xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="test.xsd">
  <Entry key="foo">bar</Config>
  <Entry key="baz">boom</Config>
</List>

• all entries are optional
• all keys have to be unique
• order of keys are irrelevant

Nothing but a key-value list.

Now I have a list of constraints like this:

• foo can only be "bar2 or "bar2"
• baz can only be "boom" or "box"

I want to put this list into the XSD, so XML-authors can not enter undefined keys or values which are not allowed for that key.

I know it would be better to get rid of the key-attribute, and make something like

<List>
  <foo>bar</foo>
  <baz>boom</foo>
</List>

But unfortunately I can not; the format must remain the same. But with more restrictions. I have to restrict the content depending to what's given in the key-attribute.

I read

• Restrict XSD attribute value based on another attribute value

Here the <xs:assert>-element is mentioned in the second answer - that could be a solution. An ugly one though, but maybe there is better one using the usual xsd-elements?

Answer 1

In XML Schema 1.1, xs:alternative can specify attribute-dependent types, as shown at your link (Restrict XSD attribute value based on another attribute value).

For your example, a schema might look like the following:

<?xml version="1.0" encoding="utf-8"?>
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema"
           xmlns:xsv="http://www.w3.org/2007/XMLSchema-versioning"
           elementFormDefault="qualified"
           xsv:minVersion="1.1">

  <xs:element name="List">
    <xs:complexType>
      <xs:sequence>
        <xs:element name="Config" minOccurs="0" maxOccurs="unbounded">
          <xs:alternative test="@key='foo'" type="ConfigKeyFooType"/>
          <xs:alternative test="@key='baz'" type="ConfigKeyBazType"/>
        </xs:element>
      </xs:sequence>
    </xs:complexType>
  </xs:element>

  <xs:attributeGroup name="ConfigTypeAttributeGroup">
    <xs:attribute name="key" type="xs:ID" use="required" />
  </xs:attributeGroup>

  <xs:complexType name="ConfigKeyFooType">
    <xs:simpleContent>
      <xs:extension base="ConfigKeyFooContentType">
        <xs:attributeGroup ref="ConfigTypeAttributeGroup"/>
      </xs:extension>
    </xs:simpleContent>
  </xs:complexType>
  <xs:simpleType name="ConfigKeyFooContentType">
    <xs:restriction base="xs:NMTOKEN">
      <xs:enumeration value="bar"/>
      <xs:enumeration value="bar2"/>
    </xs:restriction>
  </xs:simpleType>

  <xs:complexType name="ConfigKeyBazType">
    <xs:simpleContent>
      <xs:extension base="ConfigKeyBazContentType">
        <xs:attributeGroup ref="ConfigTypeAttributeGroup"/>
      </xs:extension>
    </xs:simpleContent>
  </xs:complexType>
  <xs:simpleType name="ConfigKeyBazContentType">
    <xs:restriction base="xs:NMTOKEN">
      <xs:enumeration value="boom"/>
      <xs:enumeration value="box"/>
    </xs:restriction>
  </xs:simpleType>

</xs:schema>

A valid xml…

<?xml version="1.0" encoding="utf-8"?>
<List>
  <Config key="foo">bar</Config>
  <Config key="baz">boom</Config>
</List>

…is assessed as valid.

An invalid xml due to duplicate keys…

<?xml version="1.0" encoding="utf-8"?>
<List>
  <Config key="foo">bar</Config>
  <Config key="baz">boom</Config>
  <Config key="foo">bar2</Config>
  <Config key="baz">box</Config>
</List>

…is assessed as invalid with messages:

[Error] sample-bad-dup-keys.xml:5:21:cvc-id.2: There are multiple occurrences of ID value 'foo'.
[Error] sample-bad-dup-keys.xml:5:21:cvc-attribute.3: The value 'foo' of attribute 'key' on element 'Config' is not valid with respect to its type, 'ID'.
[Error] sample-bad-dup-keys.xml:6:21:cvc-id.2: There are multiple occurrences of ID value 'baz'.
[Error] sample-bad-dup-keys.xml:6:21:cvc-attribute.3: The value 'baz' of attribute 'key' on element 'Config' is not valid with respect to its type, 'ID'.

An invalid xml due to invalid values…

<?xml version="1.0" encoding="utf-8"?>
<List>
  <Config key="foo">bar3</Config>
  <Config key="baz">boox</Config>
</List>

…is assessed as invalid with messages:

[Error] sample-bad-values.xml:3:34:cvc-enumeration-valid: Value 'bar3' is not facet-valid with respect to enumeration '[bar, bar2]'. It must be a value from the enumeration.
[Error] sample-bad-values.xml:3:34:cvc-complex-type.2.2: Element 'Config' must have no element [children], and the value must be valid.
[Error] sample-bad-values.xml:4:34:cvc-enumeration-valid: Value 'boox' is not facet-valid with respect to enumeration '[boom, box]'. It must be a value from the enumeration.
[Error] sample-bad-values.xml:4:34:cvc-complex-type.2.2: Element 'Config' must have no element [children], and the value must be valid.

Tested with Online Schema Validator service (linked from Xerces Using XML Schemas) running "Apache Xerces-J (v 2.12.2) XML Schema validator".