Documentation:Derivation expressions

You can derive both attributes and association ends.

When it comes to sub-classes that inherits classes that has anything derived you may want to override the existing derivations.

You do this in the Derivation Expressions tool:

Consider this model:

The Green side subclasses 2,3 match the pink side subclasses 5,6 - it would be good to have a derived association that match 1 and 4 whatever the subclass is.

1. Implement the derived link and set the ocl implementation to Class4.nullvalue - it is not going to be used if your Class1 is abstract
2. On class3 click up DerivationExpressions - find the Class4Derived expression - replace it with ocl: self.Class5 (valid since Class5 is a Class4)
3. On class2 click up DerivationExpressions - find the Class4Derived expression - replace it with ocl: self.Class6 (valid since Class5 is a Class4)

You have now overriden the dummy defualt implementation ( Class4.nullvalue) to something that is valid in the context of the subclass.