- From: C. M. Sperberg-McQueen <cmsmcq@blackmesatech.com>
- Date: Mon, 10 Dec 2012 14:52:39 -0700
- To: "Costello, Roger L." <costello@mitre.org>
- Cc: "C. M. Sperberg-McQueen" <cmsmcq@blackmesatech.com>, "xmlschema-dev@w3.org" <xmlschema-dev@w3.org>
On Dec 8, 2012, at 5:56 AM, Costello, Roger L. wrote: > Hi Folks, > > In XML Schema 1.1 it is possible to declare an attribute of type ID and fix its value. For example: > > <attribute name="instructor" type="ID" fixed="Professor-Johnson" /> > > Yesterday I showed that example to my class and they challenged me on the usefulness of this. > > I could not think of a single practical use case for a fixed ID attribute. > > Do you have a practical use case for a fixed ID attribute? The most obvious case I can think of is to require that a document have at most one element of a given kind, even if there may be more than one place such an element could go. If you imagine a document type which must, at some location or other, have exactly one A element, exactly one B element, and exactly one C element, but which allows each of A, B, and C in many different places, providing a fixed ID would be a simple way to ensure that there is at most one of each of these element types. You could get a similar effect by using an assertion on the document root like "count(//A) < 2 and count(//B) < 2 and count(//C) < 2". -- **************************************************************** * C. M. Sperberg-McQueen, Black Mesa Technologies LLC * http://www.blackmesatech.com * http://cmsmcq.com/mib * http://balisage.net ****************************************************************
Received on Monday, 10 December 2012 22:02:50 UTC