FUML15 — fUML should allow association ends that are not association owned

The constraint fuml_association_owns_memberEnds added to Association in 7.7.2.1 requires an association in fUML to own all its ends. While this simplifies association semantics, it is inconsistent with the default in most UML tools, which is to have association ends owned by the end types. Further, SysML requires that only non-navigable ends can be association owned (see SysML 1.5, 8.3.2.4 Block, Constraint 3).

Now, it is often sufficient to simply consider non-association-owned ends to be just regular attributes of their owning classifiers, ignoring the association, for the purpose of fUML semantics. Nevertheless, doing this loses any bidrectional semantics the association might have. And such a model is technically not fUML conformant.

Therefore, it would be better if fUML provided semantics for associations that did not own all their ends.

Allow associations with unowned ends

If there current constraint requiring associations to own their ends is simply removed, the current semantics already handles associations with unowned ends almost completely correctly. This is because it is already possible to perform structural feature actions on owned ends of associations as if they were features of the types of the opposite ends. The semantics works the same even if the ends are not owned by the association, including bidirectionality. The key to this is that if an association end is owned by an end type, then the values of that end are still maintained only in links of the association, not in feature values on instances of the end types.

There are only two additional actions that need to be considered:

• Reclassify object action: If an object is reclassified such that it losses a feature that is an association end, then all links from that object via that association end need to be destroyed.

• Unmarshall action: The values of any attribute of an unmarshalled value that is an association end needs to be obtained from the links of the appropriate association. (Unmarshall actions where added to fUML by the resolution of FUML15-13.)