I have two tables, and need help with the associations:

table_a (id int primary key, description text);

table_b(id int primary key, link_1 int foreign key (table_a id), link_2 int foreign key (table_a id) unique);

So from table_a's perspective, it's a one to many relation ship if joining on link_1 column, but a one to one when joining on link_2.

Any suggestions on what associations I can use?

So both link1 and link2 reference the table_a identifier.

Because the link2 has a unique constraint, it means that you might have multiple table_b rows with a link1 value that point to one row in table_a, but those table_b rows would need to still have unique link2 values.

You could map two @ManyToOne associations using link1 and link2 in TableB entity and have the mappedBy sides (with @OneToMany and @OneToOne) in the TableA entity.

But as soon as I do @ManyToOne in TableB Entity with @JoinColumn("link_2") and @OneToOne in TableA Entity with mappedBy="link2", Eclipse throws an error saying "In attribute EntityA, the mapped by attribute link2 has an invalid mapping type for this relation".

It seems, it's not possible to have ManyToOne on one entity and OneToOne on another entity of the relations.

Try this

Actually I might have given you the wrong suggestion, because the TableB should have a @ManyToOne for link1 and a @OneToOne for link2.
Then TableA has a OneToMany mappedBy to link1 and a @OneToOne mappedBy to link2.

So, there are not two @ManyToOne but a @ManyToOne and a @OneToOne is TableB.

Thank you, this worked despite Eclipse claiming it to be an error. Had disable JPA validation to be able to make it work.

Yes, this is what finally worked. Thanks!