How to specify Constraints in XSD (XML Schema Definition)?

Question

Problem

I want to modell the following in XSD (XML Schema definition).

I have the following relationship between three classes:

Textuell description:

• Each Widget references 0 or more Actions
• Each Trigger references one Widget and one Action
• Widgets may be referenced by 0 or more Triggers
• Actions may be referenced by 0 ore more Widgets and 0 or more Triggers
• The Action referenced by a Trigger must also be referenced by "the Triggers Widget" (and thats the problem)

In my XSD each class is modelled as a complexType.

Question

Is it possible to to model the constraint (orange note, last point) in XSD? If yes, how? In UML I'd use a Composite Structure Diagram or OCL.

Answer 1

I believe this is referred to as a co-constraint: you want to put a rule into Trigger's definition that impacts the content of two other things at the same time: something in Widget must contain something from Action.

You can't do that with XSD, but you can do it with Schematron. Schematron will let you write a rule with an XPath expression that checks a contains relationship. That Xpath expression would then be your rule that you'd check against.