Object hierarchies are very different from relational hierarchies. Relational hierarchies focus on data and its relationships, whereas objects manage not only data, but also their identity and the behavior centered around that data.

Delegated types newly introduced here looks like a Class Table Inheritance (CTI).

A very visible aspect of the object-relational mismatch is the fact that relational databases don't support inheritance. You want database structures that map clearly to the objects and allow links anywhere in the inheritance structure. Class Table Inheritance supports this by using one database table per class in the inheritance structure.

I suggest to make it UNIQUE because it seems like the column should be unique