Question

How to enforce runtime rules with XML Schema on attribute values?

Asked by: nyxll

I know this is likely possible, but I'm having trouble locating a how-to guide that explains exactly what I want to do...

I have a recursively-defined complexType (SubNode) referenced by the root (RootNode) of my schema (see code snippet).

...

Now, I have a list of defined SubType enums but even more specifically, I want to be able to have rules regarding the "type" attribute of the SubNode. Say I have type0, type1, type2 and type3 being the valid enums. But since a SubNode can have one or more SubNode child nodes, I want to specify that a type0 SubNode should have exactly one type1 SubNode child. And that a type1 SubNode should have exactly two type2 SubNode children. And a type2 SubNode should have exactly three type3 SubNode children.

I think I have to use something outside the XML Schema spec...but what? How do I enforce these rules? Schematron? I've never used it...just heard the name.

<xs:element name="root" type="RootNode"/>
 
 <xs:complexType name="RootNode">
  <xs:element name="sub" type="SubNode"/>
  <xs:attribute name="version" type="xs:string"/>
  <xs:attribute name="name" type="xs:string" use="required"/>
 </xs:complexType>
 
 <xs:complexType name="SubNode">
  <xs:choice>
    <xs:element name="sub" type="SubNode" minOccurs="0" maxOccurs="unbounded"/>
   <xs:element name="other" type="OtherNode"/>
  </xs:choice>
  <xs:attribute name="type" type="SubType" use="required"/>
  <xs:attribute name="id" type="xs:string" use="required"/>
 </xs:complexType>

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Asked On: 2009-10-02 at 17:24:18ID24781848
Tags: XML Schema attribute value runtime check schematron
Topics: Extensible Markup Language (XML)
,; XPath
Participating Experts: 1
Points: 500
Comments: 10

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Answers

by: GertonePosted on 2009-10-03 at 01:54:32ID: 25484461

This can comfortably be done by Schematron, and likely I would choose that route myself.
But, this could also be done using just W3C schema

If I understand what you need, you basically need the element subNode to have a different type, depending on the scope

Well you can define the elements locally.
As long as it will be clear to the parser which type you need at which level,
you will not hit the unique particle rule

I will post an example in a minute

by: GertonePosted on 2009-10-03 at 02:47:14ID: 25484577

depending how far you want to drive the recursivity.
This schema works up to a given depth.
If you need an infinite recursivity, you might be better of using schematron

Of course this schema can be optimised for not having all these code copying.
But that is a next step

<?xml version="1.0" encoding="UTF-8"?>
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema">
    <xs:element name="root" type="RootNode"/>
    
    <xs:complexType name="RootNode">
        <xs:sequence>
            <xs:element name="sub" type="SubNode0"/>
        </xs:sequence>
        <xs:attribute name="version" type="xs:string"/>
        <xs:attribute name="name" type="xs:string" use="required"/>
    </xs:complexType>
    
    <xs:complexType name="SubNode0">
        <xs:choice>
            <xs:element name="sub" type="SubNode1" minOccurs="1" maxOccurs="1"/>
            <xs:element name="other" type="OtherNode"/>
        </xs:choice>
        <xs:attribute name="type" type="SubType" use="required"/>
        <xs:attribute name="id" type="xs:string" use="required"/>
    </xs:complexType>
    
    <xs:complexType name="SubNode1">
        <xs:choice>
            <xs:element name="sub" type="SubNode2" minOccurs="2" maxOccurs="2"/>
            <xs:element name="other" type="OtherNode"/>
        </xs:choice>
        <xs:attribute name="type" type="SubType" use="required"/>
        <xs:attribute name="id" type="xs:string" use="required"/>
    </xs:complexType>
    
    <xs:complexType name="SubNode2">
        <xs:choice>
            <xs:element name="sub" type="SubNode3" minOccurs="3" maxOccurs="3"/>
            <xs:element name="other" type="OtherNode"/>
        </xs:choice>
        <xs:attribute name="type" type="SubType" use="required"/>
        <xs:attribute name="id" type="xs:string" use="required"/>
    </xs:complexType>
    
    <xs:complexType name="SubNode3">
        <xs:choice>
            <xs:element name="other" type="OtherNode"/>
        </xs:choice>
        <xs:attribute name="type" type="SubType" use="required"/>
        <xs:attribute name="id" type="xs:string" use="required"/>
    </xs:complexType>
    
    <xs:simpleType name="SubType">
        <xs:restriction base="xs:string"/>
    </xs:simpleType>
    <xs:simpleType name="OtherNode">
        <xs:restriction base="xs:string"/>
    </xs:simpleType>
    
</xs:schema>

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by: nyxllPosted on 2009-10-05 at 10:48:20ID: 25497550

Gertone, thanks for the answer! I see what you're doing there, and while that perfectly solves the problem if I wanted to have only SubNode0 type at the top, but my requirement is that any SubType SubNode could be at the top. I know there's no polymorphism so I can't create extensions for a SubNode base type... (My first attempt was to put a choice of multiple subNode declarations but you already answered that question for me before as being impossible to do!)

My hope was to have some sort of mechanism like <unique> (maybe <assert test="@type='SubType0'"> or something) to check the type attributes at runtime so I could validate that my instance file is following the appropriate rules of hierarchy.

So...looks like probably Schematron is my only hope at this point. I'm going to go start reading up on that but in the meantime are there some keywords or examples for how to hook it in?

Thanks!

by: GertonePosted on 2009-10-27 at 13:23:50ID: 25677017

I thought most of it is solved... I might have missed the follow on schematron, willl see if I can post something on that later today, if still relevant. There is enough valid input in the question to not simply delete it

by: GertonePosted on 2009-10-28 at 12:01:38ID: 25686937

Hi nyxll,

I must admit that I sort of misread your last reply and had a too optimistic view about that reply.
I apologize for my lack of attention

Well, I can confirm that what you want is not possible with XML schema alone.
You need to extend your schema one way or another
- with an additional schema language (schematron then comes to my mind immedeatly)
- maybe using an XSLT that you run over the XML after validation, just to report on issues (this is basically what schematron does under the hood)

There is a very good reference for extending schemas
http://www.xfront.com/ExtendingSchemas.html
sorry for not providing that earlier

I will see if I can workout an example for you.
Hope it is still usefull for you

by: nyxllPosted on 2009-10-28 at 13:38:09ID: 31636649

Thanks! Great reference provided, and that about answers my initial query. :-)

by: GertonePosted on 2009-10-28 at 17:17:10ID: 25689530

This seems to me all the schematron you need

<sch:schema xmlns:sch="http://www.ascc.net/xml/schematron">
    <sch:pattern name="number of sub element = number of sub child elements of parent + 1 or nothing">
        <sch:rule context="sub">
            <sch:assert test="count(sub) = count(preceding-sibling::sub | self::sub | following-sibling::sub) + 1 or other">count error</sch:assert>
        </sch:rule>
    </sch:pattern>
 </sch:schema>

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by: GertonePosted on 2009-10-28 at 17:19:33ID: 25689541

And this would be the schema with the above schematron integrated

<?xml version="1.0" encoding="UTF-8"?>
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema"
    xmlns:sch="http://www.ascc.net/xml/Schematron"
    >
    <xs:element name="root" type="RootNode"/>
    
    <xs:complexType name="RootNode">
        <xs:sequence>
            <xs:element name="sub" type="SubNode0"/>
        </xs:sequence>
        <xs:attribute name="version" type="xs:string"/>
        <xs:attribute name="name" type="xs:string" use="required"/>
    </xs:complexType>
    
    <xs:complexType name="SubNode0">
        <xs:choice>
            <xs:element ref="sub" minOccurs="0" maxOccurs="unbounded"/>
            <xs:element name="other" type="OtherNode" />
        </xs:choice>
        <xs:attribute name="type" type="SubType" use="required"/>
        <xs:attribute name="id" type="xs:string" use="required"/>
    </xs:complexType>
    
    <xs:element name="sub" type="SubNode0">
            <xs:annotation>
                <xs:appinfo>
                    <sch:pattern name="number of sub element = number of sub child elements of parent + 1 or nothing">
                        <sch:rule context="sub">
                            <sch:assert test="count(sub) = count(preceding-sibling::sub | self::sub | following-sibling::sub) + 1 or other">count error</sch:assert>
                        </sch:rule>
                    </sch:pattern>
                </xs:appinfo>
            </xs:annotation>
    </xs:element>
    
    
    <xs:simpleType name="SubType">
        <xs:restriction base="xs:string"/>
    </xs:simpleType>
    <xs:simpleType name="OtherNode">
        <xs:restriction base="xs:string"/>
    </xs:simpleType>
    
</xs:schema>

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by: nyxllPosted on 2009-10-28 at 18:32:07ID: 25689918

Thanks much, Gertone!!

20120131-EE-VQP-002

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How to enforce runtime rules with XML Schema on attribute values?

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XML Schema attribute value runtime check schematron

Extensible Markup Language (XML)

XPath

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