Defining composed keys [XSD]

[Francesc Guim Bernat]

Dear collegues,

I've this XML:

<?xml version="1.0"?>
<model>
<entity id="1">
<object id="1"></object>
<object id="2"></object>
</entity>
<entity id="2">
<object id="1"></object>
<object id="2"></object>
</entity>
</model>

I want to define a "composed" primary key for identify an element
object, where the identifiers of this object are the id of the entity
and the id of the object. But as i can't define a selector that point
to objects parent i'm not able to check this property throught an XML
Schema.

How i can define an XML schema that verify this property ?
any idea?


Thanks,

Francesc Guim Bernat