XSD for elements string with attribute id - xsd

Question

I would like to write a XSD element that permit something like that :
<CustomFields>
<CustomField id="1">some text data</CustomField>
<CustomField id="2">some text data</CustomField>
</CustomFields>
But I have some restrictions : I have to restrict the text (maxLenght = 36). And I would like to be impossible to have 2 CustomField with the same id.
So far I wrote this, but it not what I want :
<xs:element name="CustomFields" minOccurs="0">
<xs:complexType>
<xs:sequence>
<xs:element name="CustomField" minOccurs="0" maxOccurs="20">
<xs:complexType>
<xs:simpleContent>
<xs:extension base="xs:string">
<xs:attribute name="id" type="xs:integer" use="required"></xs:attribute>
</xs:extension>
</xs:simpleContent>
</xs:complexType>
</xs:element>
</xs:sequence>
</xs:complexType>
</xs:element>
Thanks for any help.
Regards.

Answer 1

You could use following attempt
<?xml version="1.0" encoding="UTF-8"?>
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema" elementFormDefault="qualified" attributeFormDefault="unqualified">
<xs:simpleType name="restrictedString">
<!-- Make a new type to be a "descendant" of string-->
<xs:restriction base="xs:string">
<xs:maxLength value="36"/>
</xs:restriction>
</xs:simpleType>
<xs:element name="CustomFields">
<xs:complexType>
<xs:sequence>
<xs:element name="CustomField" minOccurs="0" maxOccurs="20">
<xs:complexType>
<xs:simpleContent>
<!-- reference new type you declared above -->
<xs:extension base="restrictedString">
<xs:attribute name="id" type="xs:integer" use="required"/>
</xs:extension>
</xs:simpleContent>
</xs:complexType>
</xs:element>
</xs:sequence>
</xs:complexType>
<!-- Restrict #id to be unique-->
<xs:unique name="id_uq">
<xs:selector xpath="CustomField"/>
<xs:field xpath="#id"/>
</xs:unique>
</xs:element>
</xs:schema>