如何在休眠狀態下使用另一個表格列來映射表格列

Question

我正在使用spring rest在hibernate中創建一個Web應用程序，並且我卡在兩個表格之間。我想要映射一個表列與另一個表列。我已經在jsp中完成了，但現在我想在hibernate中完成。我從alert表中獲取author_id，然後我想從user_table中獲取該作者id的配置文件圖片。

JSP代碼：

<% 

String sql = "select author_id from alert where id=2"; 
Statement st = con.createStatement(); 
ResultSet rs = st.executeQuery(sql); 
while(rs.next()){ 
String aid = rs.getString("author_id"); 
String sql1 = "select profile_img from user_table where id='"+aid+"'"; 
Statement st1 = con.createStatement(); 
ResultSet rs1 = st1.executeQuery(sql1); 
while(rs1.next()){ 
String image_name = rs1.getString("profile_img"); 
} 
} 
%> 

我想這樣做在hibernate.Here是我Alert類

@Entity 
@Table(name="alert") 
@JsonIgnoreProperties({"hibernateLazyInitializer", "handler"}) 
public class Alert implements Serializable { 
private static final long serialVersionUID = 1L; 
@Id 
@GeneratedValue 
@Column(name="id") 
private long id; 

@Column(name="author_id") 
private long author_id; 

public long getId() { 
    return id; 
} 
public void setId(long id) { 
    this.id = id; 
} 

public long getAuthor_id() { 
    return author_id; 
} 

public void setAuthor_id(long author_id) { 
    this.author_id = author_id; 
} 

這裏是我的User類

@Entity 
@Table(name="user_table") 
@JsonIgnoreProperties({"hibernateLazyInitializer", "handler"}) 
public class User implements Serializable { 
private static final long serialVersionUID = 1L; 
@Id 
@GeneratedValue 
@Column(name="id") 
private long id; 

@Column(name="profile_img") 
private String img_name; 

public long getId() { 
    return id; 
} 

public void setId(long id) { 
    this.id = id; 
} 

public String getImg_name() { 
    return img_name; 
} 

public void setImg_name(String img_name) { 
    this.img_name = img_name; 
} 
} 

這裏是我的DAO等級：

@Autowired 
SessionFactory sessionFactory; 

Session session = null; 
Transaction tx = null; 

public User getUserById(long id) throws Exception { 
    session = sessionFactory.openSession(); 
    User user = (User) session.load(User.class, new Long(id)); 
    tx = session.getTransaction(); 
    session.beginTransaction(); 
    tx.commit(); 
    return user; 
} 

@SuppressWarnings({ "unchecked", "rawtypes" }) 
public List<Alert> getAlertById(long id) throws Exception { 
    session = sessionFactory.openSession(); 
    Criteria cr = session.createCriteria(Alert.class); 
    cr.add(Restrictions.eq("alert_viewer_id", id)); 
    List alert = cr.list(); 
    tx = session.getTransaction(); 
    session.beginTransaction(); 
    tx.commit(); 
    return alert; 
} 

請幫我，我被困在冬眠。

Answer 1

使用ORM時，您需要考慮對象及其相互關係，而不是數據庫表，列，外鍵等。雖然可以將表和列直接映射到類和字段，這樣做不會獲得太多里程，因爲將應用程序數據視爲對象圖並使用ORM管理該圖的真正目的將被擊敗。

如果更改：

@Column(name="author_id") 
private long author_id; 

public long getAuthor_id() { return author_id; } 

public void setAuthor_id(long author_id) { this.author_id = author_id; } 

到：

@JoinColumn(name = "author_id") 
@ManyToOne(fetch = FetchType.LAZY) 
private User author; 

public User getAuthor() { return author; } 

public void setAuthor(User author) { this.author = author; } 

任何時候你有一個Alert對象，你可以簡單地調用它的getAuthor得到User對象回來，你可以直接合作。

此更改不需要對數據庫進行任何更改。