Cascade下的增删改查

User.java

import javax.persistence.Entity;  
import javax.persistence.GeneratedValue;  
import javax.persistence.Id;  
import javax.persistence.ManyToOne;  
import javax.persistence.Table;  

@Entity  
@Table(name="t_user")  
public class User {  

    private int id;  
    private String name;  
    private Group group;  
    @ManyToOne(cascade={CascadeType.ALL})  
    public Group getGroup() {  
        return group;  
    }  
    public void setGroup(Group group) {  
        this.group = group;  
    }  
    @Id  
    @GeneratedValue  
    public int getId() {  
        return id;  
    }  
    public void setId(int id) {  
        this.id = id;  
    }  
    public String getName() {  
        return name;  
    }  
    public void setName(String name) {  
        this.name = name;  
    }  

}

Group.java

import java.util.HashSet;  
import java.util.Set;  

import javax.persistence.Entity;  
import javax.persistence.GeneratedValue;  
import javax.persistence.Id;  
import javax.persistence.JoinColumn;  
import javax.persistence.OneToMany;  
import javax.persistence.Table;  

@Entity  
@Table(name="t_group")  
public class Group {  

    private int id;  
    private String name;  
    private Set<User> set = new HashSet<User>();  

    @OneToMany(mappedBy="group")  
    public Set<User> getSet() {  
        return set;  
    }  
    public void setSet(Set<User> set) {  
        this.set = set;  
    }  
    @Id  
    @GeneratedValue  
    public int getId() {  
        return id;  
    }  
    public void setId(int id) {  
        this.id = id;  
    }  
    public String getName() {  
        return name;  
    }  
    public void setName(String name) {  
        this.name = name;  
    }  

}

我们在User里面添加了cascade属性，这个属性可以让我们在保存User对象的时候，会将User对象关联的对象也一并存储进去。

@ManyToOne(cascade={CascadeType.ALL})

比如

User u1 = new User();
Group g = new Group();
u1.setGroup(g)
session.save(u1)

这时候，当我们存储u1时，我们会自动把g也给存进去，但是反过来就不行了

同时，由于我们没有在Group里面设置关联，所以存g不会把它关联的u1也给存进去

User u1 = new User();
Group g = new Group();
g.getUsers.add(u1)
session.save(g)

接下来我们在两边都加上了cascade
User.java

import javax.persistence.Entity;  
import javax.persistence.GeneratedValue;  
import javax.persistence.Id;  
import javax.persistence.ManyToOne;  
import javax.persistence.Table;  

@Entity  
@Table(name="t_user")  
public class User {  

    private int id;  
    private String name;  
    private Group group;  
    @ManyToOne(cascade={CascadeType.ALL})  
    public Group getGroup() {  
        return group;  
    }  
    public void setGroup(Group group) {  
        this.group = group;  
    }  
    @Id  
    @GeneratedValue  
    public int getId() {  
        return id;  
    }  
    public void setId(int id) {  
        this.id = id;  
    }  
    public String getName() {  
        return name;  
    }  
    public void setName(String name) {  
        this.name = name;  
    }  

}

Group.java

import java.util.HashSet;  
import java.util.Set;  

import javax.persistence.Entity;  
import javax.persistence.GeneratedValue;  
import javax.persistence.Id;  
import javax.persistence.JoinColumn;  
import javax.persistence.OneToMany;  
import javax.persistence.Table;  

@Entity  
@Table(name="t_group")  
public class Group {  

    private int id;  
    private String name;  
    private Set<User> set = new HashSet<User>();  

    @OneToMany(mappedBy="group",cascade={CascadeType.ALL})  
    public Set<User> getSet() {  
        return set;  
    }  
    public void setSet(Set<User> set) {  
        this.set = set;  
    }  
    @Id  
    @GeneratedValue  
    public int getId() {  
        return id;  
    }  
    public void setId(int id) {  
        this.id = id;  
    }  
    public String getName() {  
        return name;  
    }  
    public void setName(String name) {  
        this.name = name;  
    }  

}

这时候我们使用以下方式存储g的时候，有个很遗憾的情况发生了，我们虽然会连同u1存进去，但是u1数据对应的groupid并没有值，也就是说，并没有形成关联

User u1 = new User();
Group g = new Group();
g.getUsers.add(u1)
session.save(g)

这时候我们需要添加下面这句，给它手动关联才可行

User u1 = new User();
Group g = new Group();
g.getUsers.add(u1)
u1.setGroup(g)//需要加上这句话
session.save(g)

Cascade下的增删改查

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