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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
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