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Saving list of integers with JPA?

We have a pojo that needs to have a list of integers. As an example, I created pojo Messageand would like to link the list groupIds(these identifiers should be requested and displayed in the user interface). Therefore, ideally, we would like to do something like this:

Message msg = em.find(Message.class, 101);
List<Integer> groupIds = msg.getGroupIds();

I got the impression that this would only require one pojo with JPA, but according to the discussion here I need to create a second pojo because JPA works in terms of objects instead of primitive types.

From this discussion, I tried the following code example, but getting an error openjpa-1.2.3-SNAPSHOT-r422266:907835 fatal user error: org.apache.openjpa.util.MetaDataException: The type of field "pojo.Group.messageId" isn't supported by declared persistence strategy "ManyToOne". Please choose a different strategy.

DDL:

CREATE TABLE "APP". "MESSAGE" (
  "MESSAGE_ID" INTEGER NOT NULL GENERATED ALWAYS AS IDENTITY (START WITH 1, INCREMENT BY 1),
  "AUTHOR" CHAR (20) NOT NULL
 );

ALTER TABLE "APP". "MESSAGE" ADD CONSTRAINT "MESSAGE_PK" PRIMARY KEY ("MESSAGE_ID");

CREATE TABLE "APP". "GROUP_ASSOC" (
  "GROUP_ID" INTEGER NOT NULL,
  "MESSAGE_ID" INTEGER NOT NULL
 );

ALTER TABLE "APP". "GROUP_ASSOC" ADD CONSTRAINT "GROUP_ASSOC_PK" PRIMARY KEY ("MESSAGE_ID", "GROUP_ID");

ALTER TABLE "APP". "GROUP_ASSOC" ADD CONSTRAINT "GROUP_ASSOC_FK" FOREIGN KEY ("MESSAGE_ID")
 REFERENCES "APP". "MESSAGE" ("MESSAGE_ID");

POJO,

@Entity
@Table(name = "MESSAGE")
public class Message {
    @Id
    @Column(name = "MESSAGE_ID")
    @GeneratedValue(strategy = GenerationType.IDENTITY)    
    private Long messageId;

    @OneToMany   
    private List<Group> groups = new ArrayList<Group>();

    @Column(name = "AUTHOR")
    private String author;

    // getters/setters ommitted
}    

@Entity
@IdClass(pojo.Group.GroupKey.class)
@Table(name = "GROUP_ASSOC")
public class Group {

 @Id
 @Column(name = "GROUP_ID")
 private Long groupId;

 @Id
 @Column(name = "MESSAGE_ID")
 @ManyToOne
 private Long messageId;

 public static class GroupKey {
  public Long groupId;
  public Long messageId;

  public boolean equals(Object obj) {
   if(obj == this) return true;
            if(!(obj instanceof Group)) return false;
   Group g = (Group) obj;
   return g.getGroupId() == groupId && g.getMessageId() == messageId; 
  }

  public int hashCode() {
            return ((groupId == null) ? 0 : groupId.hashCode())
                ^ ((messageId == null) ? 0 : messageId.hashCode());
  } 
 }

 // getters/setters ommitted 
}

Test code:

EntityManager em = Persistence.createEntityManagerFactory("JPATest").createEntityManager();
em.getTransaction().begin();

Message msg = new Message();
msg.setAuthor("Paul");
em.persist(msg);
List<Group> groups = new ArrayList<Group>();

Group g1 = new Group();
g1.setMessageId(msg.getMessageId());
Group g2 = new Group();
g2.setMessageId(msg.getMessageId());

msg.setGroups(groups);
em.getTransaction().commit();

It all seems ridiculous - 3 classes (if you include a GroupKey compound identity class) to model a list of integers - isn't there a more elegant solution?

0
3

, , , many-to-many ( Message Group).

DDL :

CREATE TABLE "APP"."MESSAGE" (
  "MESSAGE_ID" INTEGER NOT NULL GENERATED ALWAYS AS IDENTITY (START WITH 1, INCREMENT BY 1),
  "AUTHOR" CHAR(20) NOT NULL
 );

ALTER TABLE "APP"."MESSAGE" ADD CONSTRAINT "MESSAGE_PK" PRIMARY KEY ("MESSAGE_ID");

CREATE TABLE "APP"."GROUP" (
  "GROUP_ID" INTEGER NOT NULL GENERATED ALWAYS AS IDENTITY (START WITH 1, INCREMENT BY 1)
 );

ALTER TABLE "APP"."GROUP" ADD CONSTRAINT "GROUP_PK" PRIMARY KEY ("GROUP_ID");

CREATE TABLE "APP"."MESSAGE_GROUP" (
  "GROUP_ID" INTEGER NOT NULL,
  "MESSAGE_ID" INTEGER NOT NULL
 );

ALTER TABLE "APP"."MESSAGE_GROUP" ADD CONSTRAINT "MESSAGE_GROUP_PK" PRIMARY KEY ("MESSAGE_ID", "GROUP_ID");

ALTER TABLE "APP"."MESSAGE_GROUP" ADD CONSTRAINT "MESSAGE_GROUP_FK1" FOREIGN KEY ("MESSAGE_ID")
 REFERENCES "APP"."MESSAGE" ("MESSAGE_ID");

ALTER TABLE "APP"."MESSAGE_GROUP" ADD CONSTRAINT "MESSAGE_GROUP_FK2" FOREIGN KEY ("GROUP_ID")
 REFERENCES "APP"."MESSAGE" ("GROUP_ID");

:

@Entity
public class Message {
    @Id
    @Column(name = "MESSAGE_ID")
    @GeneratedValue(strategy = GenerationType.IDENTITY)    
    private Long messageId;

    @ManyToMany
    @JoinTable(
        name = "MESSAGE_GROUP", 
        joinColumns = @JoinColumn(name = "MESSAGE_ID"), 
        inverseJoinColumns = @JoinColumn(name = "GROUP_ID")
    ) 
    private List<Group> groups = new ArrayList<Group>();

    private String author;

    //...
}    

@Entity
public class Group {    
    @Id
    @GeneratedValue
    @Column(name = "GROUP_ID")
    private Long groupId;

    @ManyToMany(mappedBy = "groups")
    private List<Message> messages = new ArrayList<Message>();

    //...
}

, . , JPA ( , Entities). , , JPA , .

?

, "" , JPA 2.0 ElementCollection mapping ( ):

. ElementCollection " " Embeddable Basic (, ).

JPA 2.0. JPA 1.0 , . , OpenJPA @PersistentCollection.

+2

, OpenJPA2 , String. ElementCollection " ", . , , SQL.

@Entity @Table(name="user") @Access(AccessType.FIELD)
public class User {
    @Id @GeneratedValue(strategy=GenerationType.IDENTITY)
    private long id;    // primary key (autogen surrogate)
    private String name;

    // ElementCollection provides simple OneToMany linking.
    // joinColumn.name=foreign key in child table. Column.name=value in child table
    @ElementCollection(fetch=FetchType.LAZY)
    @CollectionTable(name="user_role", joinColumns={@JoinColumn(name="user_id")})
    @Column(name="role")
    private List<String> roles;

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

    public String getName() { return name; }
    public void setName(String name) { this.name=name; }

    public List<String> getRoles() { return roles; }
    public void setRoles(List<String> roles) { this.roles=roles; }

}
- - -
CREATE TABLE user (
  id bigint NOT NULL auto_increment,
  name varchar(64) NOT NULL default '',
  PRIMARY KEY (id),
  UNIQUE KEY USERNAME (name)
) ENGINE=InnoDB DEFAULT CHARSET=utf8 ;

CREATE TABLE user_role (
  user_id bigint NOT NULL,
  role varchar(64) NOT NULL default '',
  PRIMARY KEY (user_id, role)
) ENGINE=InnoDB DEFAULT CHARSET=utf8 ;
+2

ManyToOne Group Message. , , .

.

@Entity
@Table(name = "GROUP_ASSOC")
public class Group {
    @Id
    @Column(name="GROUP_ID")
    private int id;

    @ManyToOne
    @Column(name="MESSAGE_ID")
    @ForeignKey
    private Message message;

    // . . . 
}

@Entity
public class Message {
    @Id
    @GeneratedValue(strategy=GenerationType.IDENTITY)
    @Column(name = "MESSAGE_ID")
    private int id;

    @Column(length=20)
    private String author;

    @OneToMany(mappedBy="message")  
    private Collection<Group> groups;
}

IDClass ( , ).

, ,

    Query q =  em.createQuery("Select g.id from Group g where g.message.id = :messageId");
    q.setParameter("messageId", 1);

    List results = q.getResultList();

Message.getGroups():

Message m = em.find(Message.class, 1);
for(Group g : m.getGroups()) {
    // create a list, process the group whatever fits.
}
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