Best Practice: constrain an element's content by (1) a run-timeselection

["Costello, Roger L." <costello@mitre.org>]

Hi Folks,

Consider this book publication:

    <Publication kind="book">
        <Title>Everything is Miscellaneous</Title>
        <Author>David Weinberger</Author>
        <Date>2007</Date>
        <ISBN>0-8050-8811-3</ISBN>
        <Publisher>Henry Holt and Company, LLC</Publisher>
    </Publication>


Next, consider this magazine publication:

    <Publication kind="magazine">
        <Title>Science News</Title>
        <Date>2005</Date>
    </Publication>


Notice the *kind* attribute in both examples.

If its value is 'book' then the content of <Publication> is:

    - Title
    - Author
    - Date
    - ISBN
    - Publisher

And if its value is 'magazine' then the content of <Publication> is:

    - Title
    - Date



PROBLEM STATEMENT

What is best practice for constraining the content of Publication?



XML SCHEMA 1.1 PROVIDES TWO APPROACHES

XML Schema 1.1 provides two approaches to constraining the content of the <Publication> element.



APPROACH #1: ALTERNATE TYPES

Create a BookType and a MagazineType and then select one of them to be Publication's type depending on @kind:

   if @kind = 'book' then select BookType
   else select MagazineType


Here's how it is expressed in XML Schema 1.1:

   <xs:element name="Publication" type="PublicationType">
      <xs:alternative test="@kind eq 'magazine'" type="MagazineType" />
      <xs:alternative test="@kind eq 'book'" type="BookType" />
   </xs:element>


You see the (new) <alternative> element being used to select a type for Publication based on the value of @kind. 


(I don't show the complexType definition for MagazineType and BookType because it's the same as in XML Schema 1.0.)



APPROACH #2: XPATH EXPRESSION

Let the content of Publication be a collection of all the elements (both book elements and magazine elements) and set them optional: 

    - Title (0,1)
    - Author (0, unbounded)
    - Date (0,1)
    - ISBN (0,1)
    - Publisher (0,1)


Then create an XPath expression that selects the set of children for Publication depending on the value of @kind:

   if (@kind eq 'book') then
      Title and Date and ISBN and Publisher and 
          empty(* except (Title[1],Date[1],Author,ISBN[1],Publisher[1]))
   else
      if (@kind eq 'magazine') then
         Title and Date and 
           empty(* except (Title[1],Date[1]))
      else
         true()


Here's how it is expressed in XML Schema 1.1:

<xs:element name="Publication">
    <xs:complexType>
       <xs:sequence>
            <xs:element name="Title" type="xs:string"  minOccurs="0"/>
            <xs:element name="Author" type="xs:string" minOccurs="0"
                                                       maxOccurs="unbounded"/>
            <xs:element name="Date" type="xs:gYear"  minOccurs="0"/>
            <xs:element name="ISBN" type="xs:string"  minOccurs="0"/>
            <xs:element name="Publisher" type="xs:string"  minOccurs="0"/>
       </xs:sequence>
       <xs:attribute name="kind" type="xs:string" />
       <xs:assert test="if (@kind eq 'book') then
                          Title and Date and ISBN and Publisher and 
                          empty(* except (Title[1],Date[1],Author,ISBN[1],Publisher[1]))
                        else
                            if (@kind eq 'magazine') then
                               Title and Date and empty(* except (Title[1],Date[1]))
                            else
                               Title and Date and 
                                  empty(* except (Title[1],Date[1], Author))" />
    </xs:complexType>
</xs:element>


You see that the content of Publication is all the book and magazine elements and they are optional. 

You see an XPath expression within the (new) <assert> element being used to constrain which child elements are allowed within Publication based on the value of @kind.



TWO APPROACHES

You have seen two ways of solving the problem of constraining the content of Publication:

(a) Run-time selection of alternate types

(b) Run-time selection of child elements using XPath



DEFINITION OF "RUN-TIME" 

By "run-time" I mean that the content of Publication is not determined until an instance document is validated against a schema.



WHICH IS BEST PRACTICE?

Which approach is best practice? 

What are the pros and cons of each approach?


/Roger