Optional key in XSD -

i've created key/keyref on root element in order create document-wide uniqueness based on specified element.

therefore, via .//foo/@name every occurrence of @name across instances of foo must unique; likewise .//bar/@name. seems working fine. these referenced .//foo-ref/@name-ref , .//bar-ref/@name-ref respectively, defined @ root node.

however, i've gathered 1 cannot create optional key, , presenting bit of problem. semantically, nature of conforming documents, key not required on every single instance of foo or bar. instances of foo-ref/@name-ref need target existing foo/@name, isn't semantically invalid foo without @name.

is there work-around this? don't idea of consumers having define key every single element, when reasonably handful need them.

here's example schema (of course, i'm not deploying foobar schema, structure identical; testing schema i've been toying with)

<xs:schema elementformdefault="qualified" xmlns:xs="http://www.w3.org/2001/xmlschema">     <xs:complextype name="ref">         <xs:attribute name="name-ref" type="xs:string" use="required" />     </xs:complextype>     <xs:complextype name="obj">         <xs:attribute name="name" type="xs:string" use="optional" />     </xs:complextype>     <xs:complextype name="foo">         <xs:complexcontent>             <xs:extension base="obj">                 <xs:sequence>                     <xs:choice maxoccurs="unbounded">                         <xs:element name="foo" type="foo" />                         <xs:element name="bar" type="bar" />                         <xs:element name="foo-ref" type="foo-ref" />                         <xs:element name="bar-ref" type="bar-ref" />                     </xs:choice>                 </xs:sequence>             </xs:extension>         </xs:complexcontent>     </xs:complextype>     <xs:complextype name="foo-ref">         <xs:complexcontent>             <xs:extension base="ref" />         </xs:complexcontent>     </xs:complextype>     <xs:complextype name="bar">         <xs:complexcontent>             <xs:extension base="obj">                 <xs:sequence>                     <xs:choice maxoccurs="unbounded">                         <xs:element name="bar" type="bar" />                         <xs:element name="qux" type="qux" />                         <xs:element name="bar-ref" type="bar-ref" />                     </xs:choice>                 </xs:sequence>             </xs:extension>         </xs:complexcontent>     </xs:complextype>     <xs:complextype name="bar-ref">         <xs:complexcontent>             <xs:extension base="ref" />         </xs:complexcontent>     </xs:complextype>     <xs:complextype name="qux">         <xs:simplecontent>             <xs:extension base="xs:string" />         </xs:simplecontent>     </xs:complextype>     <xs:element name="root">         <xs:complextype>             <xs:sequence>                 <xs:element name="foo" type="foo" maxoccurs="unbounded" />             </xs:sequence>         </xs:complextype>         <xs:key name="foo">             <xs:selector xpath=".//foo" />             <xs:field xpath="@name" />         </xs:key>         <xs:key name="bar">             <xs:selector xpath=".//bar" />             <xs:field xpath="@name" />         </xs:key>         <xs:keyref name="foo-ref" refer="foo">             <xs:selector xpath=".//foo-ref" />             <xs:field xpath="@name-ref" />         </xs:keyref>         <xs:keyref name="bar-ref" refer="bar">             <xs:selector xpath=".//bar-ref" />             <xs:field xpath="@name-ref" />         </xs:keyref>     </xs:element> </xs:schema> 

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addendum

just following revisions @petrugardea. unique can referenced keyref, knew? (not me apparently)

<xs:element name="root">     <xs:complextype>         <xs:sequence>             <xs:element name="foo" type="foo" maxoccurs="unbounded" />         </xs:sequence>     </xs:complextype>     <xs:keyref name="foo-ref" refer="foo">         <xs:selector xpath=".//foo-ref" />         <xs:field xpath="@name-ref" />     </xs:keyref>     <xs:keyref name="bar-ref" refer="bar">         <xs:selector xpath=".//bar-ref" />         <xs:field xpath="@name-ref" />     </xs:keyref>     <!--         use of xs:unique here, in lieu of xs:key allows         nullable "keys", retaining referential integrity         above defined keyrefs. awesome possum.     -->     <xs:unique name="foo">         <xs:selector xpath=".//foo" />         <xs:field xpath="@name" />     </xs:unique>     <xs:unique name="bar">         <xs:selector xpath=".//bar" />         <xs:field xpath="@name" />     </xs:unique> </xs:element> 

use xsd:unique; unlike key, matched value either unique or nil (nil or not present).

example:

<?xml version="1.0" encoding="utf-8" ?> <!-- xml schema generated qtassistant/xsd module (http://www.paschidev.com) --> <xsd:schema xmlns:xsd="http://www.w3.org/2001/xmlschema">     <xsd:element name="root">         <xsd:complextype>             <xsd:sequence>                 <xsd:element name="uk" maxoccurs="unbounded">                     <xsd:complextype>                         <xsd:attribute name="name" type="xsd:string"/>                     </xsd:complextype>                 </xsd:element>                 <xsd:element name="fk" maxoccurs="unbounded">                     <xsd:complextype>                         <xsd:attribute name="name" type="xsd:string"/>                     </xsd:complextype>                 </xsd:element>             </xsd:sequence>         </xsd:complextype>         <xsd:unique name="uq">             <xsd:selector xpath="uk"/>             <xsd:field xpath="@name"/>         </xsd:unique>         <xsd:keyref name="fk" refer="uq">             <xsd:selector xpath="fk"/>             <xsd:field xpath="@name"/>         </xsd:keyref>     </xsd:element> </xsd:schema> 

sample (valid) xml:

<?xml version="1.0" encoding="utf-8" standalone="yes"?> <!-- sample xml generated qtassistant (http://www.paschidev.com) --> <root xmlns:xsi="http://www.w3.org/2001/xmlschema-instance">     <uk name="name1"/>     <uk />     <fk/>     <fk name="name1"/> </root>