2007-11-14

用JoinColumn使用JPA的OneToMany不自动生成数据库表

有两个实体Order和Customer，一个用户可以有多个订单，一个订单属于一个用户，所以Customer与Order是一对多的关系，那么在Order实体里可以有一个到Customer的引用customer，同时在Customer里有一个Order实体的集合orders。如果在只是如下这样写：

public class Order {
    @id
    private long id;
 
    @ManyToOne
    private Customer customer;
 
    public void setId(long id) {
        this.id = id;
    }
 
    public long getId() {
        return this.id;
    }
 
    public void setCustomer(Customer customer) {
        this.customer = customer;
    }
 
    public Customer getCustomer() {
        return this.customer;
    }
}
 
public class Customer{
    @id
    private long id;
 
    @OneToMany
    private Set<Order> orders;
 
    public void setId(long id) {
        this.id = id;
    }
 
    public long getId() {
        return this.id;
    }
 
    public void setOrders(Set<Order> orders) {
        this.orders = orders;
    }
 
    public Set<Order> getOrders() {
        return this.orders;
    }
}

那么系统可能会自动生成一张Customer_Order关联表，可是事实上我们不需要这样的表，因为通过 Select order from Order order where order.customer.id=:id， 就可以找到某个Customer的Orders，而不需要另建一张表。要想去掉这个自动生成的表，要这样写：

public class Order {
    @id
    private long id;
 
    @ManyToOne
    @JoinColumn(name="customer", nullable=false)
    private Customer customer;
 
    public void setId(long id) {
        this.id = id;
    }
 
    public long getId() {
        return this.id;
    }
 
    public void setCustomer(Customer customer) {
        this.customer = customer;
    }
 
    public Customer getCustomer() {
        return this.customer;
    }
}
 
public class Customer{
    @id
    private long id;
 
    @OneToMany(mappedBy="customer")
    private Set<Order> orders;
 
    public void setId(long id) {
        this.id = id;
    }
 
    public long getId() {
        return this.id;
    }
 
    public void setOrders(Set<Order> orders) {
        this.orders = orders;
    }
 
    public Set<Order> getOrders() {
        return this.orders;
    }
}

多出一个mappedBy和JoinColumn, 就是告诉JPA实现者，orders是通过order里的customer映射来的，每次查找orders通过Order里的customer联接。这样就不会生成Customer_Order关联表了

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