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I am using standalone Papyrus Version: 2022-12 (4.26) Build id: 2022-12-15T14:23:43Z This report concerns a subtlety in the correct usage of UML constraints, and the possible conflation of the meanings of {unique} and {id} in the Papyrus tool. In the Papyrus class diagram editor, attribute properties may be declared {unique} - the default setting - and this seems to be the only way to mark any kind of unique identifying property. I cannot find {id} anywhere. The UML {unique} constraint strictly applies to collections, not attributes (viz. the constraints {unique, ordered} together specify all four OCL collection types), and {unique} means that every element of a collection is unique (it is a set, not a bag). If applied to an attribute, I infer the meaning that every instance of the class has a unique value for this attribute (viz. it is a candidate key). It should be possible instead to specify the UML constraint {id} on one or more attributes of the same class, to indicate that these attributes participate in a (possibly compound) primary key. I cannot find a way to do this in the Papyrus CD editor without violating EMF syntax check constraints. Being able to specify {id} would be more useful, and more general, than specifying {unique}. Supporting only {unique} and not {id} rules out the possibility of compound keys; and in any case {unique} describes a candidate key (a field that could, but need not, be chosen to represent the primary key if there are other candidates). This is a subtle point of UML modelling that took me years to figure out; and I hope that I have made this point clear. It would be great if Papyrus could support {id} in the sense described.
