XSD - Extend XSD then add custom nested elements

Question

I am trying to design a XSD for 2 different XML message coming in:

<message>
    <common1>value1</common1>
    <common2>value2</common2>
    <common3>
        <unique1>hello</unique1>
    </common3>
    <common4>
        <unique2>bye</unique2>
    </common4>
</message>

or

<message>
    <common1>value1</common1>
    <common2>value2</common2>
    <common3>
        <unique3>hey</unique3>
    </common3>
    <common4>
        <unique4>goodbye</unique4>
    </common4>
</message>

Since i see that they have common elements and only unique nested elements, i want to create a generic parent that an xsd can extend so i can just worry about the unique fields but i still don't have an idea on how to define common3 or 4 in parent level so i have defined it on the child xsds which i think is wrong, can anyone verify and also shed any light on this?

parent.xsd:
<?xml version="1.0"?>
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema">
    <xs:element name="message">
      <xs:complexType>
        <xs:sequence>
          <xs:element name="common1" type="xs:string"/>
          <xs:element name="common2" type="xs:string"/>
        </xs:sequence>
      </xs:complexType>
    </xs:element>
</xs:schema>

and

child1.xsd:
<?xml version="1.0"?>
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema">
    <xs:include schemaLocation="parent.xsd"/>
    <xs:complexType>
        <xs:complexContent>
            <xs:extension base="message">
                <xs:sequence>
                    <xs:element name="common3" type="common3Type" />
                    <xs:element name="common4" type="common4Type" />
                </xs:sequence>
            </xs:extension>
        </xs:complexContent>
    </xs:complexType>
    <xs:complexType name="common3Type">
      <xs:sequence>
          <xs:element name="unique1" type="xs:string"/>
      </xs:sequence>
    </xs:complexType>
    <xs:complexType name="common4Type">
      <xs:sequence>
          <xs:element name="unique2" type="xs:string"/>
      </xs:sequence>
    </xs:complexType>
</xs:schema>

and

child2.xsd:
<?xml version="1.0"?>
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema">
    <xs:include schemaLocation="parent.xsd"/>
    <xs:complexType>
        <xs:complexContent>
            <xs:extension base="message">
                <xs:sequence>
                    <xs:element name="common3" type="common3Type" />
                    <xs:element name="common4" type="common4Type" />
                </xs:sequence>
            </xs:extension>
        </xs:complexContent>
    </xs:complexType>
    <xs:complexType name="common3Type">
      <xs:sequence>
          <xs:element name="unique3" type="xs:string"/>
      </xs:sequence>
    </xs:complexType>
    <xs:complexType name="common4Type">
      <xs:sequence>
          <xs:element name="unique4" type="xs:string"/>
      </xs:sequence>
    </xs:complexType>
</xs:schema>

Use choice instead of extending, it will be simpler. https://www.w3schools.com/xml/el_choice.asp — sashwat, May 24 '18 at 04:18

score 2 · Answer 1 · answered May 24 '18 at 08:31

You can use extension-by-restriction with XML Schema such that you have a generic definition for common3 and common4 in the super-type, and then constrain those definitions for your child1 and child2 message sub-types, respectively:

<!-- message.xsd -->
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema">

  <xs:complexType name="message">
    <xs:sequence>
      <xs:element name="common1" type="xs:string"/>
      <xs:element name="common2" type="xs:string"/>
      <xs:element name="common3" type="xs:anyType" minOccurs="0"/>
      <xs:element name="common4" type="xs:anyType" minOccurs="0"/>
    </xs:sequence>
  </xs:complexType>

  <xs:complexType name="message-subtype-a">
    <xs:complexContent>
      <xs:restriction base="message">
        <xs:sequence>
          <xs:element name="common1" type="xs:string"/>
          <xs:element name="common2" type="xs:string"/>
          <xs:element name="common3" type="common3-subtype-a" minOccurs="1"/>
          <xs:element name="common4" type="common4-subtype-a" minOccurs="1"/>
        </xs:sequence>
      </xs:restriction>
    </xs:complexContent>
  </xs:complexType>

  <xs:complexType name="message-subtype-b">
    <xs:complexContent>
      <xs:restriction base="message">
        <xs:sequence>
          <xs:element name="common1" type="xs:string"/>
          <xs:element name="common2" type="xs:string"/>
          <xs:element name="common3" type="common3-subtype-b" minOccurs="1"/>
          <xs:element name="common4" type="common4-subtype-b" minOccurs="1"/>
        </xs:sequence>
      </xs:restriction>
    </xs:complexContent>
  </xs:complexType>

  <xs:complexType name="common3-subtype-a">
    <xs:sequence>
      <xs:element name="unique1" type="xs:string"/>
    </xs:sequence>
  </xs:complexType>

  <xs:complexType name="common4-subtype-a">
    <xs:sequence>
      <xs:element name="unique2" type="xs:string"/>
    </xs:sequence>
  </xs:complexType>

  <xs:complexType name="common3-subtype-b">
    <xs:sequence>
      <xs:element name="unique3" type="xs:string"/>
    </xs:sequence>
  </xs:complexType>

  <xs:complexType name="common4-subtype-b">
    <xs:sequence>
      <xs:element name="unique4" type="xs:string"/>
    </xs:sequence>
  </xs:complexType>

</xs:schema>

Note I've put all type definitions in a single schema file here. For declaring elements you still need two schemas for the respective message sub-types, since you want the messages to be both contained in a message element:

<!-- message-a.xsd -->
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema">
  <xs:include schemaLocation="message.xsd"/>
  <xs:element name="message" type="message-subtype-a"/>
</xs:schema>

and

<!-- message-b.xsd -->
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema">
  <xs:include schemaLocation="message.xsd"/>
  <xs:element name="message" type="message-subtype-b"/>
</xs:schema>

When using derivation-by-restriction, you must repeat the content model for each individual sub-type, as explained in XML Schema Restriction for Complex types : Complete Redefinition? by one of the authors of the XML Schema specification.

hello, thanks for your reply, will this also be feasible for scenarios that we have unlimited unknown version of the common3/4 instead of 2 version? if not, what else would you suggest? the ideal solution would hopefully allow any xsd to extend everything and just have their own unique sub elements under common3 and common4, thanks! — R.C, May 24 '18 at 13:05
I'd suggest to check out XML Schema *wildcards* which are basically particles in your content model that validate *any* element when occuring in that position (or any declared element, or any element in a particular namespace, etc.); see eg. https://stackoverflow.com/questions/943659/how-can-i-define-an-xsd-file-that-allows-unknown-wildcard-elements — imhotap, May 24 '18 at 14:24

XSD - Extend XSD then add custom nested elements

1 Answers1