什么是@joincolumn以及如何在Hibernate中使用它

Question

我读了很多关于@JoinColumn的文章，但我仍然不明白它背后的想法。

病人床

CREATE TABLE patient (
patient_id BIGINT NOT NULL,
first_name VARCHAR(255) NOT NULL,
last_name VARCHAR(255) NOT NULL,
PRIMARY KEY(patient_id));

车辆表

CREATE TABLE vehicles (
patient_id BIGINT NOT NULL,
vehicle_id BIGINT NOT NULL,
vehicle_manufacturer VARCHAR(255),
PRIMARY KEY (vehicle_id),
CONSTRAINT patienthasmanyvehicle FOREIGN KEY(patient_id) REFERENCES patient(patient_id));

@OneToMany(mappedBy = "patient")
    private Collection<Vehicle> patientVehicles = new ArrayList<Vehicle>();

@ManyToOne
@JoinColumn(name="patient_id")
private Patient patient;

Vehicle Class ---- Entity
@JoinColumn(name="patient_id") ---- annotation
private Patient patient ----field

我一直在读这篇文章，但我仍然感到困惑。JPA JoinColumn与mappedBy

Answer 1

通过联接表的单向关联

@Entity
class Patient {

    @OneToMany
    private Collection<Vehicle> vehicles = new ArrayList<Vehicle>();

}

@Entity
class Vehicle {

}

通过联接表的双向关联

@Entity
class Patient {

    @OneToMany
    private Collection<Vehicle> vehicles = new ArrayList<Vehicle>();

}

@Entity
class Vehicle {

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

}

通过外键的单向关联

@Entity
class Patient {

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

}

@Entity
class Vehicle {

}

@Entity
class Patient {

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

}

@Entity
class Vehicle {

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

}

通过具有外部列名规范的外键进行的双向关联

@Entity
class Patient {

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

}

@Entity
class Vehicle {

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

}

这是使用@joincolumn的基本出发点。

要验证外键(vehicle表中的patient_id)是否真的映射到patients表中，可以使用@joincolumn(nullable=false)

@Entity
class Vehicle {

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

}