有人可以在JPA和Hibernate中解释MapdBy吗? [英] Can someone explain mappedBy in JPA and Hibernate?

问题描述

我刚冬眠，需要使用一对多和多对一关系.这是我对象中的双向关系，因此我可以从任一方向来回移动.建议使用mappedBy进行处理，但是我听不懂.有人可以解释:

I am new to hibernate and need to use one-to-many and many-to-one relations. It is a bi-directional relationship in my objects, so that I can traverse from either direction. mappedBy is the recommended way to go about it, however, I couldn't understand it. Can someone explain:

• 推荐的使用方式是什么?
• 它能解决什么目的?

就我的示例而言，这是我的带有注释的类:

For the sake of my example, here are my classes with annotations:

• Airline 拥有很多 AirlineFlights
• 很多 AirlineFlights属于 ONE Airline

• Airline OWNS many AirlineFlights
• Many AirlineFlights belong to ONE Airline

航空公司:

@Entity 
@Table(name="Airline")
public class Airline {
    private Integer idAirline;
    private String name;

    private String code;

    private String aliasName;
    private Set<AirlineFlight> airlineFlights = new HashSet<AirlineFlight>(0);

    public Airline(){}

    public Airline(String name, String code, String aliasName, Set<AirlineFlight> flights) {
        setName(name);
        setCode(code);
        setAliasName(aliasName);
        setAirlineFlights(flights);
    }

    @Id
    @GeneratedValue(strategy=GenerationType.IDENTITY)
    @Column(name="IDAIRLINE", nullable=false)
    public Integer getIdAirline() {
        return idAirline;
    }

    private void setIdAirline(Integer idAirline) {
        this.idAirline = idAirline;
    }

    @Column(name="NAME", nullable=false)
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = DAOUtil.convertToDBString(name);
    }

    @Column(name="CODE", nullable=false, length=3)
    public String getCode() {
        return code;
    }
    public void setCode(String code) {
        this.code = DAOUtil.convertToDBString(code);
    }

    @Column(name="ALIAS", nullable=true)
    public String getAliasName() {
        return aliasName;
    }
    public void setAliasName(String aliasName) {
        if(aliasName != null)
            this.aliasName = DAOUtil.convertToDBString(aliasName);
    }

    @OneToMany(fetch=FetchType.LAZY, cascade = {CascadeType.ALL})
    @JoinColumn(name="IDAIRLINE")
    public Set<AirlineFlight> getAirlineFlights() {
        return airlineFlights;
    }

    public void setAirlineFlights(Set<AirlineFlight> flights) {
        this.airlineFlights = flights;
    }
}

航空公司航班:

@Entity
@Table(name="AirlineFlight")
public class AirlineFlight {
    private Integer idAirlineFlight;
    private Airline airline;
    private String flightNumber;

    public AirlineFlight(){}

    public AirlineFlight(Airline airline, String flightNumber) {
        setAirline(airline);
        setFlightNumber(flightNumber);
    }

    @Id
    @GeneratedValue(generator="identity")
    @GenericGenerator(name="identity", strategy="identity")
    @Column(name="IDAIRLINEFLIGHT", nullable=false)
    public Integer getIdAirlineFlight() {
        return idAirlineFlight;
    }
    private void setIdAirlineFlight(Integer idAirlineFlight) {
        this.idAirlineFlight = idAirlineFlight;
    }

    @ManyToOne(fetch=FetchType.LAZY)
    @JoinColumn(name="IDAIRLINE", nullable=false)
    public Airline getAirline() {
        return airline;
    }
    public void setAirline(Airline airline) {
        this.airline = airline;
    }

    @Column(name="FLIGHTNUMBER", nullable=false)
    public String getFlightNumber() {
        return flightNumber;
    }
    public void setFlightNumber(String flightNumber) {
        this.flightNumber = DAOUtil.convertToDBString(flightNumber);
    }
}

数据库架构:

AirlineFlights具有idAirline作为ForeignKey，而Airline没有idAirlineFlights.这使得AirlightFlights成为所有者/识别实体?

AirlineFlights has the idAirline as ForeignKey and Airline has no idAirlineFlights. This makes, AirlineFlights as the owner/identifying entity ?

从理论上讲，我希望航空公司成为airlightFlights的所有者.

Theoretically, I would like airline to be the owner of airlineFlights.

推荐答案

通过在两个模型上都指定@JoinColumn，您就没有双向关系.您有两个单向关系，并且在那上面有一个非常令人困惑的映射.您要告诉这两个模型拥有" IDAIRLINE列.实际上，实际上其中只有一个！ 正常"的事情是将@JoinColumn完全移出@OneToMany一侧，而将mapledBy添加到@OneToMany.

By specifying the @JoinColumn on both models you don't have a two way relationship. You have two one way relationships, and a very confusing mapping of it at that. You're telling both models that they "own" the IDAIRLINE column. Really only one of them actually should! The 'normal' thing is to take the @JoinColumn off of the @OneToMany side entirely, and instead add mappedBy to the @OneToMany.

@OneToMany(cascade = CascadeType.ALL, mappedBy="airline")
public Set<AirlineFlight> getAirlineFlights() {
    return airlineFlights;
}

这告诉Hibernate:在我有一个用于查找配置的集合上，检查名为'airline'的bean属性."

That tells Hibernate "Go look over on the bean property named 'airline' on the thing I have a collection of to find the configuration."

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