RE: [xml-dev] Schema constraints from a different file?

[Vladimir Gapeyev <vgapeyev@seas.upenn.edu>]

On Sat, 22 Oct 2005, Michael Kay wrote:

>>
>> Basically, I would like to develop schemas that impose
>> constraints on an element or attribute in one XML file based on
>> element or attribute values from another XML file.  Is this at
>> all possible?  I would consider making a file containing just a
>> list of all available item IDs, if that would help solve the
>> problem.
>
> This can't be done directly: schema validation is confined to a single
> document. The only thing you can do is write a transformation that
> constructs a synthetic document and validate that; but then you might as
> well do the validation within the stylesheet that performs the
> transformation.

Theoretically, I think this could do the trick:

   (1) Include the XML document that contains IDs into the document that
       refers to them via a parsed external entity [XML Rec, 4.2.2, 4.3]

   (2) Use WXS key/keyref feature [WXS, Part 1, 3.11] to encode your
       referential constraints.

(1) will physically keep the data in separate files, but the processor 
should pull them together, so the schema (2) could be written as if the 
data was present in a single file.

Are there reasons this wouldn't work in practice?

Vladimir