Bind an attribute to an other existing attribute - xsd

Question

i want to create a XSD.
One optional element in the XSD is <PrintoutSettings OrderSource="NameOfOrder_X" .../>
Another substructure of the XSD is
<Order Name="NameOfOrder">...</Order>
<Order Name="NameOfOrder2">...</Order>
...
My aim is it, that the attribute NameOfOrder_X of <PrintoutSettings .../> must be a string that was defined in one of the <Order>...</Order> - elements.
How can i realize it in a XSD?
Kind regards
sb
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Here some of my XSD:
<xs:complexType name="Order">
<xs:sequence>
<xs:element form="qualified" minOccurs="0" name="Documents" type="Documents"/>
</xs:sequence>
<xs:attribute name="Name" type="xs:string" use="required"/>
<xs:attribute name="Translator" type="xs:string" use="required"/>
<xs:attribute name="Proofreader" type="xs:string" use="required"/>
<xs:attribute name="LockedBy" type="xs:string" use="optional"/>
</xs:complexType>
<xs:complexType name="Orders">
<xs:sequence>
<xs:element maxOccurs="unbounded" name="Order" type="Order"/>
</xs:sequence>
</xs:complexType>
<xs:complexType name="QQDBData">
<xs:sequence>
<xs:element minOccurs="0" name="Orders" type="Orders"/>
<xs:element minOccurs="0" name="LocalSettings" type="Settings"/>
</xs:sequence>
<xs:attribute default="2000-01-01" name="CreationDate" type="xs:date" use="optional"/>
</xs:complexType>
<xs:element name="QQDBData" type="QQDBData">
<xs:key name="PK-Orders">
<xs:selector xpath="Order"/>
<xs:field xpath="#Name"/>
</xs:key>
<xs:keyref name="FK-PrintoutSettings" refer="PK-Orders">
<xs:selector xpath="PrintoutSettings"/>
<xs:field xpath="#OrderSource"/>
</xs:keyref>
</xs:element>

Answer 1

The answer you've got from #DevNull is technically correct (+1); it is, however, not so much XSD; have a look at this section from the schema primer to understand the limitations you have with ID/IDREF.
I'll take the schema put forward by DevNull and modify it to show you the recommended XSD approach that relies on key/keyref.
UPDATE: I took your schema and updated to make sure it is sufficient for my tests:
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema" elementFormDefault="qualified">
<xs:complexType name="Order">
<xs:sequence>
<xs:element form="qualified" minOccurs="0" name="Documents" type="Documents"/>
</xs:sequence>
<xs:attribute name="Name" type="xs:string" use="required"/>
<xs:attribute name="Translator" type="xs:string" use="required"/>
<xs:attribute name="Proofreader" type="xs:string" use="required"/>
<xs:attribute name="LockedBy" type="xs:string" use="optional"/>
</xs:complexType>
<xs:complexType name="Documents"/>
<xs:complexType name="Orders">
<xs:sequence>
<xs:element maxOccurs="unbounded" name="Order" type="Order"/>
</xs:sequence>
</xs:complexType>
<xs:complexType name="QQDBData">
<xs:sequence>
<xs:element minOccurs="0" name="Orders" type="Orders"/>
<xs:element minOccurs="0" name="LocalSettings" type="Settings"/>
</xs:sequence>
<xs:attribute default="2000-01-01" name="CreationDate" type="xs:date" use="optional"/>
</xs:complexType>
<xs:complexType name="Settings">
<xs:attribute name="OrderSource" use="required" type="xs:string"/>
</xs:complexType>
<xs:element name="QQDBData" type="QQDBData">
<xs:key name="PK-Orders">
<xs:selector xpath="Orders/Order"/>
<xs:field xpath="#Name"/>
</xs:key>
<xs:keyref name="FK-PrintoutSettings" refer="PK-Orders">
<xs:selector xpath="LocalSettings"/>
<xs:field xpath="#OrderSource"/>
</xs:keyref>
</xs:element>
</xs:schema>
What I had to do is to update the selectors, since by introducing additional tags, the XPath has changed. I always try to visualize the constraints, to ensure that the XPaths as described still make sense.
An updated sample XML:
<?xml version="1.0" encoding="utf-8" standalone="yes"?>
<!-- Sample XML generated by QTAssistant (http://www.paschidev.com) -->
<QQDBData xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" CreationDate="2000-01-01">
<Orders>
<Order Name="Name1" Translator="Translator1" Proofreader="Proofreader1" LockedBy="LockedBy1">
<Documents/>
</Order>
<Order Name="Name1" Translator="Translator1" Proofreader="Proofreader1" LockedBy="LockedBy1">
<Documents/>
</Order>
</Orders>
<LocalSettings OrderSource="OrderSource1"/>
</QQDBData>
An error message, for the same invalid XML may look like (I am using QTAssistant to validate):
Error occurred while loading [], line 12 position 3
The key sequence 'OrderSource1' in Keyref fails to refer to some key.

Answer 2

You can use an attribute type of xs:ID for <Order> and an attribute type of xs:IDREF for <PrintoutSettings>.
Example XSD:
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema" elementFormDefault="qualified">
<xs:element name="doc">
<xs:complexType>
<xs:sequence>
<xs:element maxOccurs="unbounded" ref="Order"/>
<xs:element ref="PrintoutSettings"/>
</xs:sequence>
</xs:complexType>
</xs:element>
<xs:element name="Order">
<xs:complexType>
<xs:simpleContent>
<xs:extension base="xs:NMTOKEN">
<xs:attribute name="Name" use="required" type="xs:ID"/>
</xs:extension>
</xs:simpleContent>
</xs:complexType>
</xs:element>
<xs:element name="PrintoutSettings">
<xs:complexType>
<xs:attribute name="OrderSource" use="required" type="xs:IDREF"/>
</xs:complexType>
</xs:element>
</xs:schema>
Example of INVALID XML: (Gives the error "There is no ID/IDREF binding for IDREF 'NameOfOrder_X'." in Xerces.)
<doc xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:noNamespaceSchemaLocation="book.xsd">
<Order Name="NameOfOrder">...</Order>
<Order Name="NameOfOrder2">...</Order>
<PrintoutSettings OrderSource="NameOfOrder_X"/>
</doc>
Example of VALID XML:
<doc xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:noNamespaceSchemaLocation="book.xsd">
<Order Name="NameOfOrder">...</Order>
<Order Name="NameOfOrder2">...</Order>
<PrintoutSettings OrderSource="NameOfOrder2"/>
</doc>