OCL bond on operation between two classes

Question

I have a class diagram and I need to write some OCL about it but I'm not sure about the right syntax. I've searched a lot and found a lot of different ways, so I'd like to know if the way I'm going to write them is correct.

For example, in my class diagram I have the classes Ticket and Event linked by the role ticket validFor event. The ticket has a price attribute which is float. Is this way correct to write OCL to calcualte the total incoming of an event?

CONTEXT Event::totalIncoming():float
post result=ticket.allInstances()->select(t : t.validFor=self)->collect(price)->sum()

I'm taking all insances of tickets, selecting all instances valid for the event, collecting their prices and finally summing them.

Answer 1

The example you provide uses Complete OCL; a self standing text document, often with a *.ocl file extension in which an Essential OCL expression is given a surrounding context as indicated by Section 12 of the OCL specification: "The Use of OCL Expressions in UML Models". This approach is helpful if you want to separate your constraint and model concerns and/or exploit the utility of bulk text editing.

Alternatively you may use a tool that provides a text box for each UML Constraint. In this case you often enter just an Essential OCL expression that the tool wraps up in an appropriate way as a Constraint.body.

If working with Ecore, you may use OCLinEcore to embed Essential OCL expressions within the *.ecore or *.oclinecore.

You example is wrong in that you should use lower case context, and a : and optionally a name after post.

Your example is probably wrong in that you use ticket rather than Ticket the class name.

Your example is undesirable in that you use allInstances which should be avoided wherever possible if you actually plan to execute the OCL.

However perhaps you did mean ticket as the collection member variable, in which case the allInstances() is wrong. ticket->select(...) is sufficient to select some of the ticket collection.

I suspect that result = ticket->select(validFor=self).price->sum() would do. However the reverse testing of validFor could be redundant since the forward navigation is often sufficient with a check of the reverse navigation redundant.