One-To-Many Relationship with Join Table

Question

I have a one-to-many relationship modeled using join table:

create table t1 (id int primary key, name varchar(10) /*...*/); create table t2 (id int primary key, name varchar(10) /*...*/); create table t1_t2 (t1_id int, t2_id int, primary key (t1, t2)); 

The tables are supposed to model the relationship of one t1 to many t2. What is the right way to model these tables using JPA?

Answer 1

The typical table for one T1 to many T2 is to have a foreign key on T2 pointing toward T1. The T1_T2 table is usually not needed.

The JPA structure would then be a One-To-Many, possibly two-way.

---

There could be some arrangements, to make the structure you describe work. You could change T1_T2:

• add a unique constraint on T2 (so that only one T2 is allowed)

Is that really what you want?

Edited: yes, it is what you want ;-)

I doubt you may find many examples on the net. I have no proved solution, but I would try something along these lines:

In Hibernate annotation reference documentation, see "2.2.5.3.2.3. Unidirectional with join table" to get the idea. It looks like:

    @Entity     public class Trainer {         @OneToMany         @JoinTable(             name="TrainedMonkeys",             joinColumns = @JoinColumn( name="trainer_id"),             inverseJoinColumns = @JoinColumn( name="monkey_id")         )         public Set<Monkey> getTrainedMonkeys() {         ...     }