JPA Definition of a One-To-Many Relationship with Junction Table

Question

I have a one-to-many relationship modeled using an extra 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 mode these tables using JPA?

Answer 1

Hi

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.

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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() {
        ...
    }