Modelling unique relationships in UML class diagrams
This question is about UML class diagrams in the analysis phase. I've made up a minimal example for illustration purposes:
A ticket is valid for one seat and one specific seminar. A seat can be assigned to multiple tickets, but only to one unique ticket/seminar combination. (The same goes for the seminar and a unique ticket/seat combination.)
The diagram does not reflect that. I've marked the wrong multiplicities in red.
What options (the more the better) do you know to model the requirement correctly?
That kind of commutative consistency check cannot be expressed either by UML or Entity Relationship diagrams. By the way, you can't express either that, from a ticket instance, the path ticket -> seat -> seminar location must lead to the same instance as ticket -> seminar -> seminar location.
Formally, the multiplicities you marked in red are correct. A seat sees more than one ticket, just for different seminars, and the other way round.
So the only solution I can offer is to include the requirement in the textual specification of those relationships. That's not a bad thing, as any finite set of graphical symbols could only express a subset of all possible requirements.
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