Controlling position of optional elements in an XML instance

[kosek]

Hi,
I'm struggling with a problem. I will try to simplify it. Let's assume we are editing the following XML:

<data>
    <a>A</a>
    <b>
        <x>XXX</x>
        <y>YYY</y>
    </b>                    
    <c>C</c>
</data>

But schema also allows optional element <z> inside <b> element. Thus there is a control for editing of this element that is bound to the proper node in XML document:

<fx-control ref="b/z">
  <label>Z:</label>
  <input>
</fx-control>

The question is: if user will fill value for <z> element where this element will appear in the instance? Is there any way how to control where new node <z> will be attached?

[JoernT]

Question: you mention a schema for the XML. Does it require a certain position in the data? In the given case your z element will be the last child of <b> i guess.

Just asking as often the there's no special restriction on the order of elements in the XML data and therefore does not matter.

Controlling that position is currently not explicitly possible i think though that would need a test to verify.

Did i at least get the problem right?

[kosek]

Yes, schema requires fixed ordering of elements. This is very common in XML and most XML vocabularies require particular ordering. (The main reason is that while XML Schema allows unordered content models by using xs:all compositor it is very rarely used in the practice as it has many other limitations. So most schemas are using xs:sequence that defines one and only one correct order of elements.)

[JoernT]

for xs:sequence in a schema that certainly true.

Fore was not designed as a schema-aware editor. However this does not mean it can't be done - just not out-of-the-box. There are several ideas to make it work though it has not been a priority right now. We'll discuss approaches and get back to you.

I convert this to a discussion as this seems more appropriate.

[rkottmann]

Option 1 (already working):

one already supplies the optional element in the instance:

 <data>
    <a>A</a>
    <b>
        <x>XXX</x>
        <y>YYY</y>
       <!-- empty and let's say currently invalid -->
       <z></z>
    </b>                    
    <c>C</c>
</data>

Option 2 (just brainstorming):

<!-- to place element z between x and y -->
<fx-control ref="b[x]/z" after="x">
  <label>Z:</label>
  <input>
</fx-control>

where any xpath in after attribute must be an already existing child element of parent of z.

[kosek]

I think that option 2 is viable approach in some scenarios. Based on experience from other XML update languages (XQuery, Schematron QuickFix) several options for specifying position of newly inserted element are needed:

• insert-before
• insert-after
• insert-into
• insert-into-as-first
• insert-into-as-last

But even such mechanism is not always sufficient. Assume that all of elements x, y and z are optional and must appear in that order inside b element.

For x and z we can use:

<fx-control ref="b/x" insert-into-as-first="b">
  <label>X:</label>
  <input>
</fx-control>

<fx-control ref="b/z" insert-into-as-last="b">
  <label>Z:</label>
  <input>
</fx-control>

But what about y? As both preceding and following siblings are optional we can't anchor y against them.

More complex solution could be introduced, supporting multiple insert positions to be specified and base their use on conditions. But that would lead to a very complex expressions, hard to create, debug and manage unless generated automatically from the schema.

Another idea would be to supply Fore with empty "template document" that would contain all elements from the schema and Fore will use this only to get information about where particular node should be inserted in actually edited instance. This is basically similar to option 1, but there will be no need to clutter instance with empty (and sometime invalid or undesired) elements. Just my 0.02 €.

[DrRataplan]

This is now resolved in Fore 2.27.0 by using the fx-bind elements to determine order. Given a schema like a? (b?|c?) d? you can have a set of binds like this:

<fx-bind ref="root">
  <fx-bind ref="a" ></fx-bind>
  <fx-bind ref="b"></fx-bind>
  <fx-bind ref="c"></fx-bind>
  <fx-bind ref="d"></fx-bind>
</fx-bind>

These binds will then be used as a first choice to place elements: a d will always be inserted after a c, b, a, whichever it finds first. Falling back on control order if they could not be found.