Uni-directionalOnly one side ("owner") knows of the relationship
Uses the @ManyToMany annotation
Defines mapping to database
Must use @JoinTable
Bi-directionalBoth classes know of the relationship
One side required to be owning side and maps relation to the database
Uses the @ManyToMany annotation
Must use @JoinTable
Changes here change the database
One side required to be inverse and names the other entity's property
@ManyToMany(mappedBy="owning-property")
Changes here do *not* change database
Figure 62.3. Many-to-Many Uni-directional Database Schema
create table ORMREL_WANTED (
+----> id bigint generated by default as identity,
| primary key (id)
| )
| create table ORMREL_WANTED_MEDIA (
`----- ORMREL_WANTED_id bigint not null,
+----- media_MEDIA_ID bigint not null
| )
| create table ORMREL_MEDIA (
`----> MEDIA_ID bigint generated by default as identity,
title varchar(255),
primary key (MEDIA_ID)
)
alter table ORMREL_WANTED_MEDIA
add constraint FKEE528304A70D4E48
foreign key (media_MEDIA_ID)
references ORMREL_MEDIA
alter table ORMREL_WANTED_MEDIA
add constraint FKEE52830486AFC6B6
foreign key (ORMREL_WANTED_id)
references ORMREL_WANTED
Figure 62.4. Many-to-Many Uni-directional Database Java Mapping
@Entity @Table(name="ORMREL_WANTED")
public class WantList {
@Id @GeneratedValue
private long id;
@ManyToMany
@JoinTable(name="ORMREL_WANTED_MEDIA") //define table, let columns use default names
+----- private Collection<Media> media = new ArrayList<Media>();
| ...
| @Entity @Table(name="ORMREL_MEDIA")
| public class Media {
| @Id @GeneratedValue @Column(name="MEDIA_ID")
`----> private long id;
private String title;
Figure 62.5. Many-to-Many Uni-directional Example Usage
for(WantList w: wantLists) {
for(Media m: media) {
//we can only navigate this in one direction
w.getMedia().add(m);
log.info("added media(" + m.getId() +
") to want list (" + w.getId() +")");
}
}
Figure 62.6. Many-to-Many Uni-directional Database Output
-added media(1) to want list (1)
-added media(2) to want list (1)
-added media(1) to want list (2)
-added media(2) to want list (2)
-added media(1) to want list (3)
-added media(2) to want list (3)
Hibernate:
insert into ORMREL_WANTED_MEDIA (ORMREL_WANTED_id, media_MEDIA_ID)
values (?, ?)
...Rows added to join table rather than updating entity class tables with foreign key
Figure 62.7. Many-to-Many Bi-directional Database Schema
create table ORMREL_AUTHOR (
+-----> id bigint generated by default as identity,
| name varchar(255),
| primary key (id)
| )
| create table ORMREL_AUTHOR_MEDIA (
`----- LINK_AUTHOR_ID bigint not null,
+----- LINK_MEDIA_ID bigint not null
| )
| create table ORMREL_MEDIA (
`----> MEDIA_ID bigint generated by default as identity,
title varchar(255),
primary key (MEDIA_ID)
)
alter table ORMREL_AUTHOR_MEDIA
add constraint FKA0D2B4E09B01C6F2
foreign key (LINK_MEDIA_ID)
references ORMREL_MEDIA
alter table ORMREL_AUTHOR_MEDIA
add constraint FKA0D2B4E089FFE922
foreign key (LINK_AUTHOR_ID)
references ORMREL_AUTHOR
Figure 62.8. Many-to-Many Bi-directional Database Java Mapping
@Entity @Table(name="ORMREL_AUTHOR")
public class Author {
@Id @GeneratedValue
private long id;
private String name;
@ManyToMany
@JoinTable(name="ORMREL_AUTHOR_MEDIA", //defines the link table
//defines the column in the link table for author FK
joinColumns={@JoinColumn(name="LINK_AUTHOR_ID")},
//defines the column in the link table for the media FK
inverseJoinColumns={@JoinColumn(name="LINK_MEDIA_ID")})
+--> @OrderBy("title DESC") //order the list returned from database
+----- private List<Media> media = new ArrayList<Media>();
| |
| | @Entity @Table(name="ORMREL_MEDIA")
| | public class Media {
| | @Id @GeneratedValue @Column(name="MEDIA_ID")
| | private long id;
| | private String title;
| `---- @ManyToMany(mappedBy="media") //names property in Author that points to us
`-----> private Collection<Author> authors = new ArrayList<Author>();
Figure 62.9. Many-to-Many Bi-directional Example Usage
for(Author a: authors) {
for(Media m: media) {
a.getMedia().add(m);
log.info("added media(" + m.getId() +
") to author (" + a.getId() +")");
m.getAuthors().add(a);
log.info("added author(" + a.getId() +
") to media (" + m.getId() +")");
}
}
Owning and inverse sides now both need to be set
Figure 62.10. Many-to-Many Bi-directional Database Output
-added media(3) to author (1)
-added author(1) to media (3)
...
-added media(5) to author (5)
-added author(5) to media (5)
Hibernate:
insert into ORMREL_AUTHOR_MEDIA (LINK_AUTHOR_ID, LINK_MEDIA_ID)
values (?, ?)
...
Hibernate:
insert into ORMREL_AUTHOR_MEDIA (LINK_AUTHOR_ID, LINK_MEDIA_ID)
values (?, ?)