[英]Many-to-many relation returns empty collection
I am trying to create two entities which have many-to-many relation between them. 我正在尝试创建两个实体,它们之间具有多对多关系。 First entity is Person with PID as primary key, second is Serie with SID as primary key.
第一个实体是将PID作为主键的Person ,第二个是将SID作为主键的Serie 。 In database there is a table TJV_5_SERIE_2_PERSON , which represents many to many relationship between these entities.
在数据库中,有一个表TJV_5_SERIE_2_PERSON ,表示这些实体之间的多对多关系。
The problem is when I retrieve any entity, Collection annotated with @ManyToMany is always empty. 问题是当我检索任何实体时,用@ManyToMany注释的Collection始终为空。 So I assume I've messed up something in my code that explains why my many-to-many relation doesn't work.
因此,我假设我在代码中弄乱了一些内容,以解释为什么我的多对多关系不起作用。
I retrieve these two entities by generating (in Netbeans 9.0) 'Restful Web Services from Entity classes'. 我通过在Netbeans 9.0中生成“来自实体类的Restful Web Services”来检索这两个实体。 This way I can use these services to retrieve all attributes succesfully, except Collection with @ManyToMany annotation is always empty.
这样,我可以使用这些服务成功检索所有属性,但带有@ManyToMany批注的Collection始终为空。
Any idea why it is not woking appreciated. 任何想法,为什么它不被唤醒赞赏。 It is first time trying this, so pardon me for any dumm mistakes.
这是第一次尝试,请原谅任何愚蠢的错误。
Person class: 人类:
@Entity
@Table(name = "TJV_5_PERSON")
@XmlRootElement
public class Person implements Serializable {
@Id
@GeneratedValue(strategy = GenerationType.AUTO)
@Column(name = "PID")
private Integer id;
@Column(name = "PNAME")
private String name;
@ManyToMany()
@JoinTable(
name = "TJV_5_SERIE_2_PERSON",
joinColumns = @JoinColumn(name = "PID", referencedColumnName = "PID"),
inverseJoinColumns = @JoinColumn(name = "SID", referencedColumnName = "SID")
)
// always empty
private Collection<Serie> favourites = new ArrayList<Serie>();
public Person() {
}
public Person(Integer id, String name) {
this.id = id;
this.name = name;
}
public Integer getId() {
return id;
}
public void setId(Integer id) {
this.id = id;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
@XmlTransient
public Collection<Serie> getFavourites() {
return favourites;
}
public void setFavourites(Collection<Serie> favourites) {
this.favourites = favourites;
}
@Override
public int hashCode() {
int hash = 5;
hash = 31 * hash + Objects.hashCode(this.id);
return hash;
}
@Override
public boolean equals(Object obj) {
if (this == obj) {
return true;
}
if (obj == null) {
return false;
}
if (getClass() != obj.getClass()) {
return false;
}
final Person other = (Person) obj;
if (!Objects.equals(this.id, other.id)) {
return false;
}
return true;
}
@Override
public String toString() {
return "Person{" + "id=" + id + ", name=" + name + ", favourites=" + favourites + '}';
}
}
Serie class: 意甲课:
@Entity
@Table(name = "TJV_5_SERIE")
@XmlRootElement
public class Serie implements Serializable {
@Id
@GeneratedValue(strategy = GenerationType.AUTO)
@Column(name = "SID")
private Integer id;
@Column(name = "STITLE")
private String title;
// always empty
@ManyToMany(mappedBy = "favourites")
private Collection<Person> fans = new ArrayList<Person>();
public Serie() {
}
public Serie(Integer id, String title) {
this.id = id;
this.title = title;
}
public Integer getId() {
return id;
}
public void setId(Integer id) {
this.id = id;
}
public String getTitle() {
return title;
}
public void setTitle(String title) {
this.title = title;
}
@XmlTransient
public Collection<Person> getFans() {
return fans;
}
public void setFans(Collection<Person> fans) {
this.fans = fans;
}
@Override
public int hashCode() {
int hash = 3;
hash = 67 * hash + Objects.hashCode(this.id);
return hash;
}
@Override
public boolean equals(Object obj) {
if (this == obj) {
return true;
}
if (obj == null) {
return false;
}
if (getClass() != obj.getClass()) {
return false;
}
final Serie other = (Serie) obj;
if (!Objects.equals(this.id, other.id)) {
return false;
}
return true;
}
@Override
public String toString() {
return "Serie{" + "id=" + id + ", title=" + title + ", fans=" + fans + '}';
}
}
I am not 100% sure, but you may not retrieving any results beacuse of @XMLTransiet annotation above the Serie.class method 我不确定100%,但是您可能无法通过Serie.class方法上方的@XMLTransiet注释检索任何结果
@XmlTransient
public Collection<Person> getFans() {
return fans;
}
Try to look in documentation https://docs.oracle.com/javaee/6/api/javax/xml/bind/annotation/XmlTransient.html or in connected posts Hide an entity variable from xml message - @XmlTransient not working 尝试查看文档https://docs.oracle.com/javaee/6/api/javax/xml/bind/annotation/XmlTransient.html或相关文章隐藏xml消息中的实体变量-@XmlTransient不起作用
The other issue is cascading data between two corresponding @ManyToMany tables. 另一个问题是在两个对应的@ManyToMany表之间级联数据。 It means that you have intersection and the data appears in this table automatically when you use some type of cascade but you need send a POST request.
这意味着您具有交集,并且当您使用某种类型的级联但需要发送POST请求时,数据会自动显示在此表中。 It means in your service class layer you can create a method responsible for creating Person and assign a Serie to this Person object which is a foreign key.
这意味着在您的服务类层中,您可以创建一个负责创建Person的方法,并为此外键将此Person对象分配一个Serie。 The article about cascading is here :) https://vladmihalcea.com/a-beginners-guide-to-jpa-and-hibernate-cascade-types/
关于级联的文章在这里:) https://vladmihalcea.com/a-beginners-guide-to-jpa-and-hibernate-cascade-types/
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