I am confusing how to design a OOP model for FAN and FOLLOW, just like the FRIEND and FOLLOW IN facebook.

My design:



The disadvantage is that I cannot get the SET of FAN easily.
But Is it really necessary for the relationship to design two sort of SET? (fanSet followSet)

You should have a single table person, and then for friends and follows you have two separate tables in the database.

The friends table has a composite PK build out of 2 FK: from_person_id and to_person_id. Both these two are FK to the person id PK.

The follows table is the same: the PK is also composite PK out of 2 FK: from_person_id and to_person_id. Both these two are FK to the person id PK.

Check out this article about how you can map a composite identifier with JPA and Hibernate. From the FK relationships, you can build the @ManyToOne sides.

You don't necessarily need the collections since you can map the Follow and Friend associations and just use queries for those. For more info, check out this article about @OneToMany association as well.

Instead of having two tables, I think that why not just add one more attribute to the composite identifier Object to identify this record whether follow or fan.

from_person_id to_person_id tag
1 3 follow
3 1 friend

That will work as well.