Question

There is a way to create a rule like ‘foreign key restricted’ in xsd?
I need to check the existence of an attribute value that is equal to the attribute ‘use’ of a node.

<node use="..." />
<otherNode name="..." />

See the example above, the attribute is set to ‘node’ attribute ‘use’, the value of this attribute must be equal to ‘otherNode’ in the ‘name’ attribute

Answer 1

Set the type of the use attribute to xs:IDREF

and the name attribute to xs:ID.

There’s also an xs:IDREFS type.

An example

<xsd:element name="quote">
  <xsd:complexType>
    <!--content model-->
    <xsd:attribute name="ref" type="xsd:IDREF"/>
  </xsd:complexType>
</xsd:element>
<xsd:element name="footnote">
  <xsd:complexType>
   <!--content model-->
   <xsd:attribute name="id" type="xsd:ID" use="required"/>
 </xsd:complexType>

…

Gives

<quote ref="fn1">...</quote>
<footnote id="fn1">...</footnote>

So quote must have a ref parameter, which in this case points to a footnote

Hmm would appear that the above was for DTD and doesn’t work however I found this

True key representation

If you have ever attempted to describe a relational database with a complex relationship map using a DTD, you’ve likely had to use the ID-IDREF pointing mechanism. For example, in a structure where two entities are related in a many-to-many way through a relating table (borrowers and assets on a loan application, for example), the simple XML parent-child relationship is insufficient. However, IDs and IDREFs have their own weaknesses: IDs must be unique across an entire document, and IDREF declarations do not specify the type of element an instance of the IDREF attribute must reference. XML Schema provides a way to specify these pointing relationships in much the same way that foreign-key relationships are declared in a relational database. For example, say you have a foreign-key relationship that you can’t express using a simple parent-child relationship in our XML. You can declare the two related elements as in Listing 3:

<xsd:element name="rootElement">
  <xsd:complexType>
    <xsd:sequence>
      <xsd:element name="elementOne" maxOccurs="unbounded">
        <xsd:complexType>
          <xsd:attribute name="elementOneKey" type="integer" />
          <xsd:attribute name="elementOneDesc" type="text" />
        </xsd:complexType>
        <xsd:key name="elementOnePK">
          <xsd:selector xpath=".//elementOne"/>
          <xsd:field xpath="@elementOneKey"/>
        </xsd:key>
      </xsd:element>
      <xsd:element name="elementTwo" maxOccurs="unbounded">
        <xsd:complexType>
          <xsd:attribute name="elementTwoKey" type="integer" />
          <xsd:attribute name="elementOneKey" type="integer" />
          <xsd:attribute name="elementTwoDesc" type="text" />
        </xsd:complexType>
        <xsd:keyref name="elementOneFK" refer="elementOnePK">
          <xsd:selector xpath=".//elementTwo"/>
          <xsd:field xpath="@elementOneKey"/>
        </xsd:keyref>
      </xsd:element>
     </xsd:sequence>
   </xsd:complexType>
</xsd:element>

Which is even better as you can link id and reference properly.

Have a plus I’m learning stuff.