[jboss-jira] [JBoss JIRA] Created: (BPEL-272) allow xpath expression/query to create more than one occurrence of an element

[Alejandro Guizar (JIRA)]

allow xpath expression/query to create more than one occurrence of an element
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                 Key: BPEL-272
                 URL: http://jira.jboss.com/jira/browse/BPEL-272
             Project: JBoss jBPM BPEL
          Issue Type: Feature Request
      Security Level: Public (Everyone can see)
          Components: Engine
    Affects Versions: jBPM BPEL 1.1 beta 3
            Reporter: Alejandro Guizar
         Assigned To: Alejandro Guizar
            Priority: Critical
             Fix For: jBPM BPEL 1.1 GA


PROBLEM The current XPath sublanguage implementation automatically creates nodes missing from location paths during assignment. This facility has a number of conceptual limitations. One of them is the inability to append a child element to an existing element if the latter already has a child element with the same name. There is no XPath syntax to express that a new element is required!

It is tempting to think of a numeric XPath predicate as an array index, leading to the next snippet:
<copy>
 <from><literal>haha</literal></from>
 <to variable='laughter' xmlns:ex='http://jbpm.org/bpel/examples'>
  <query>ex:laugh[last() + 1]</query>
 </to>
</copy>

However, such an interpretation of a predicate is wrong. Consider the following extract from XPath 1.0 specification on predicates:
"A PredicateExpr is evaluated by evaluating the Expr and converting the result to a boolean. If the result is a number, the result will be converted to true if the number is equal to the context position and will be converted to false otherwise."

The description of the last() function reads:
"The last function returns a number equal to the context size from the expression evaluation context."

Noticed the problem? The context position is never greater than the context size. Therefore the predicate [last() + 1] does not represent the position "one past the context size" but the boolean value false. Unfortunately, no other predicate seems appropriate. 

PROPOSAL Introduce a minor deviation from the XPath 1.0 specification. Interpret the result of a numeric predicate that equals the context size plus one as a  request to create a new occurrence of the node.

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