The trick is there are additional attributes of actions and achievements such as a category that must match for the link to be valid. These attributes are not part of the primary key of either record and can and do change.
So your data is denormalized? (The "category" appears in 2 tables?) Don't do that. Create a view that joins your two tables together instead if you need a single entity that contains data from multiple sources. Then you won't have any of the data integrity issues you're worried about.
