Optional key in XSD - xsd

Question

I've created a key/keyref on the root element in order to create document-wide uniqueness based on the specified element.
Therefore, via .//foo/#name every occurrence of #name across all instances of foo must be unique; likewise for .//bar/#name. This seems to be working fine. These are referenced by .//foo-ref/#name-ref and .//bar-ref/#name-ref respectively, also defined at the root node.
However, I've gathered that one cannot create an optional key, and this is presenting a bit of a problem. Semantically, by the nature of the conforming documents, a key is not required on every single instance of foo or bar. The instances of foo-ref/#name-ref would obviously need to target an existing foo/#name, but it isn't semantically invalid for a foo to be without a #name.
Is there any work-around for this? I don't like the idea of consumers having to define a key for every single element, when reasonably only a handful will need them.
Here's the example schema (of course, I'm not deploying some foobar schema, but the structure is identical; this is just a 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>
Addendum
Just following up with my revisions thanks to #PetruGardea. So unique can be referenced by a keyref, who 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>
<!--
the use of xs:unique here, in lieu of xs:key allows for
nullable "keys", retaining referential integrity with the
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>

Answer 1

Use xsd:unique; unlike a key, its matched value is either unique or nil (nil or not present).
Example:
<?xml version="1.0" encoding="utf-8" ?>
<!-- XML Schema generated by 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 by QTAssistant (http://www.paschidev.com) -->
<root xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
<uk name="name1"/>
<uk />
<fk/>
<fk name="name1"/>
</root>