xml-dev - Re: Should we care about elements order ?

[Ronald Bourret <rpbourret@rpbourret.com>]

Daniel Barclay wrote:

> How can sibling order be unimportant?  Don't you need to preserve the
> sibling order in XML to preserve the order in Java?  (You don't know if
> the Java order is significant, so you have to preserve it to be an
> accurate serialization of the Java data.)

As my example showed (repeated below), two documents with two different
sibling orders can be used to populate the same object:

   <Part>
      <Number>123</Number>
      <Desc>Turkey wrench</Desc>
      <Price>10.95</Price>
   </Part>

and:

   <Part>
      <Price>10.95</Price>
      <Desc>Turkey wrench</Desc>
      <Number>123</Number>
   </Part>

both populate:

   object part {
      number = 123
      desc = "Turkey wrench"
      price = 10.95
   }

The part object has no notion of an order among number, desc, and price.

Where sibling order might be important for objects is when more than one
sibling maps to the *same* property. For example, multiple LineItem
children of a SalesOrder would map to an array of line items:

   class SalesOrder {
      int number;
      Date date;
      Customer cust;
      LineItem[] lineItems;
   }

This assumes that the property is ordered (an array). I believe that
some OO languages allow you to have unordered, multi-valued properties,
such as sets.

Note that in any case this order isn't necessarily transferrable to a
relational database. This is because there is no notion of order among
rows in a table. (The child LineItem elements would be stored in a
separate table, as is the case for any multi-valued property.)

> (Or do all the serialization methods you've seen explicitly add an index
> so that XML sibling order no longer matters?)

You have to either add an index or designate an attribute/child element
as the index. In the example above, each line item probably has a
number, so you could use that as the index, but there is no guarantee
that such an attribute/child element will be present.

Please note that the only product that I'm sure that does this is my own
(XML-DBMS). I seem to remember seeing an order property in the DAD
mapping language in DB2. I've never seen any mention of sibling order in
white papers or web pages discussing other products.

Note that in the above case, you can use an ORDER BY clause to specify
sibling order when the mapping from XML to database is done with a
SELECT statement. However, this requires there to be a data field which
can be used to order children of the same type, which is not always
true.

-- 
Ronald Bourret
Programming, Writing, and Training
XML, Databases, and Schemas
http://www.rpbourret.com