hibernate 表设计，关系维护说明 mappedby

hibernate 表关系的维护 主要通过如@oneToOne等标签标记。在这些关系中，重要的是mappedby的设置。

如：
 

@OneToOne(cascade = { CascadeType.PERSIST, CascadeType.MERGE,     
    CascadeType.REMOVE }, mappedBy="user", fetch = FetchType.LAZY)
	@JoinColumn(name = "location_id", referencedColumnName = "id")
	private UserLocation location;
	
@OneToOne(cascade = { CascadeType.PERSIST, CascadeType.MERGE, 
    CascadeType.REMOVE }, mappedBy="user", fetch = FetchType.LAZY)
	@JoinColumn(name = "userinfo_id", referencedColumnName = "id")
	private Userinfo userinfo;
	
@OneToMany(cascade = { CascadeType.PERSIST, CascadeType.REFRESH, 
   CascadeType.MERGE }, mappedBy = "user", fetch = FetchType.LAZY)
	@OrderBy("id asc")
	private List<Album> albums;
  

这三个属性，都设置了mappedBy，则说明他和这三个实体UserLocation， Userinfo， Album之间的关系由这三个实体中来维护，‘维护’的意思是说，这三个实体会出现与user的关联关系字段，既user_id（字段名称取决于mappedBy对应user在三个实体中对user_id起的具体名字）。 同时，user_id这个关系字段的增加、删除、修改均由三个实体发起，不能由User发起。
   如：
  

public Userinfo getUserinfo() {
		if (userinfo == null){
			userinfo = new Userinfo();//CascadeType.PERSIST
			userinfo.setUser(this);//mappedby user, set "user_id" in userinfo
		}
		return userinfo;
	}

public List<Album> getAlbums() {
		if (albums == null){
			Album headAlbum = new Album();
			headAlbum.setUser(this);
			
			albums = new ArrayList<Album>();
			albums.add(headAlbum);
		}
		return albums;
	}
      
public UserLocation getLocation() {
		if (location == null){
			location = new UserLocation();
			location.setUser(this);
		}
		return location;
	}

在三个实体中分别定义User

UserLocation：
	@OneToOne(cascade = { CascadeType.REFRESH, CascadeType.MERGE }, fetch = FetchType.LAZY)
	@JoinColumn(name = "user_id", referencedColumnName = "id")
	private User user;

UserInfo：
@OneToOne(cascade = { CascadeType.REFRESH, CascadeType.MERGE }, fetch = FetchType.LAZY)
	@JoinColumn(name = "user_id", referencedColumnName = "id")
	private User user;

Album：
@OneToMany(cascade = { CascadeType.PERSIST, CascadeType.MERGE, CascadeType.REMOVE }, fetch = FetchType.LAZY, mappedBy = "album", targetEntity = UserPhoto.class)
	@OrderBy("rank asc")
	private List<UserPhoto> photos;


然后对这些属性进行设置：
//transfer UserinfoDto to userinfo
		if (userinfo != null){
			userinfo.toObject(user.getUserinfo());
		}

		// create user photo wall Album
		user.getAlbums();
		
		if (location != null) {
			location.toObject(user.getLocation());
		}

如果去掉user中 userinfo中的定义 mappedBy="user"，则各自维护自己的关系，那么
user表中会有userinfo_id值，userinfo中有user_id. 当设置了user.setUserinfo(new Userinfo()), 则会在user中设置userinfo_id, 设置了userinfo.setUser(user)，则对应。

标签@JoinColumn(name = "userinfo_id", referencedColumnName = "id") ，userinfo_id为在user表中外键，对应userinfo表中的id主键。如果不需要在user中有userinfo的id，则删除该标签