XML Schema: require an element to be present when another is present - xsd

Question

My Schema allows a specific element to be optional, but when I encounter a different also optional element later on in the file, I need the previous one to be present. How can I ensure this via XSD?
Example:
<settings>
<file name="narf.txt"/>
<metafile name="narf.meta"/>
<filtermeta>true</filtermeta>
</settings>
should be valid,
<settings>
<file name="narf.txt"/>
<metafile name="narf.meta"/>
</settings>
and
<settings>
<file name="narf.txt"/>
</settings>
should be valid too,
<settings>
<file name="narf.txt"/>
<filtermeta>true</filtermeta>
</settings>
should not be valid.

Answer 1

Try the following schema definition:
<?xml version="1.0" encoding="UTF-8"?>
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema" elementFormDefault="qualified">
<xs:element name="settings">
<xs:complexType>
<xs:sequence>
<xs:element name="file">
<xs:complexType>
<xs:attribute name="name" type="xs:string"/>
</xs:complexType>
</xs:element>
<xs:sequence minOccurs="0">
<xs:element name="metafile">
<xs:complexType>
<xs:attribute name="name" type="xs:string"/>
</xs:complexType>
</xs:element>
<xs:element minOccurs="0" name="filtermeta" type="xs:string"/>
</xs:sequence>
</xs:sequence>
</xs:complexType>
</xs:element>
</xs:schema>
I've run your first three examples against this schema using xmllint and they validate correctly. The fourth example fails with:
cvc-complex-type.2.4.a: Invalid content was found starting with element 'filtermeta'. One of '{metafile}' is expected.
as expected.

Answer 2

You could have filtermeta as an optional attribute to the metafile element.