what is @JoinColumn and how it is used in Hibernate

Question

I have been reading a lot about @JoinColumn but I still don\'t get the idea behind it.

Patient Table

CREATE TABLE patient (
patient_id BIGINT NOT NUL         


        

Answer 1

A unidirectional association via a join table

@Entity
class Patient {

    @OneToMany
    private Collection vehicles = new ArrayList();

}

@Entity
class Vehicle {

}

A bidirectional association via a join table

@Entity
class Patient {

    @OneToMany
    private Collection vehicles = new ArrayList();

}

@Entity
class Vehicle {

    @ManyToOne(fetch = FetchType.LAZY)
    private Patient patient;

}

A unidirectional association via a foreign key

@Entity
class Patient {

    @OneToMany
    @JoinColumn
    private Collection vehicles = new ArrayList();

}

@Entity
class Vehicle {

}

A bidirectional association via a foreign key

@Entity
class Patient {

    @OneToMany(mappedBy = "patient")
    private Collection vehicles = new ArrayList();

}

@Entity
class Vehicle {

    @ManyToOne(fetch = FetchType.LAZY)
    private Patient patient;

}

A bidirectional association via a foreign key with a foreign column name specification

@Entity
class Patient {

    @OneToMany(mappedBy = "patient")
    private Collection vehicles = new ArrayList();

}

@Entity
class Vehicle {

    @ManyToOne(fetch = FetchType.LAZY)
    @JoinColumn(name="patient_id")
    private Patient patient;

}

This is the basic starting point of using @JoinColumn.

To verify that the foreign key(patient_id in the Vehicle table) is really mapped in the patients table you can use @JoinColumn(nullable = false)

@Entity
class Vehicle {

    @ManyToOne(fetch = FetchType.LAZY)
    @JoinColumn(name="patient_id", nullable = false)
    private Patient patient

}