@ManyToMany associations
Most often, you will need to use @JoinTable annotation to specify the mapping of a many-to-many table relationship:
- the name of the link table and
- the two Foreign Key columns
So, assuming you have the following database tables:
In the Post entity, you would map this relationship, like this:
@ManyToMany(cascade = {
CascadeType.PERSIST,
CascadeType.MERGE
})
@JoinTable(
name = "post_tag",
joinColumns = @JoinColumn(name = "post_id"),
inverseJoinColumns = @JoinColumn(name = "tag_id")
)
private List<Tag> tags = new ArrayList<>();
The @JoinTable annotation is used to specify the table name via the name attribute, as well as the Foreign Key column that references the post table (e.g., joinColumns) and the Foreign Key column in the post_tag link table that references the Tag entity via the inverseJoinColumns attribute.
Notice that the cascade attribute of the
@ManyToManyannotation is set toPERSISTandMERGEonly because cascadingREMOVEis a bad idea since we the DELETE statement will be issued for the other parent record,tagin our case, not to thepost_tagrecord.
Unidirectional @OneToMany associations
The unidirectional @OneToMany associations, that lack a @JoinColumn mapping, behave like many-to-many table relationships, rather than one-to-many.
So, assuming you have the following entity mappings:
@Entity(name = "Post")
@Table(name = "post")
public class Post {
@Id
@GeneratedValue
private Long id;
private String title;
@OneToMany(
cascade = CascadeType.ALL,
orphanRemoval = true
)
private List<PostComment> comments = new ArrayList<>();
//Constructors, getters and setters removed for brevity
}
@Entity(name = "PostComment")
@Table(name = "post_comment")
public class PostComment {
@Id
@GeneratedValue
private Long id;
private String review;
//Constructors, getters and setters removed for brevity
}
Hibernate will assume the following database schema for the above entity mapping:
As already explained, the unidirectional @OneToMany JPA mapping behaves like a many-to-many association.
To customize the link table, you can also use the @JoinTable annotation:
@OneToMany(
cascade = CascadeType.ALL,
orphanRemoval = true
)
@JoinTable(
name = "post_comment_ref",
joinColumns = @JoinColumn(name = "post_id"),
inverseJoinColumns = @JoinColumn(name = "post_comment_id")
)
private List<PostComment> comments = new ArrayList<>();
And now, the link table is going to be called post_comment_ref and the Foreign Key columns will be post_id, for the post table, and post_comment_id, for the post_comment table.
Unidirectional
@OneToManyassociations are not efficient, so you are better off using bidirectional@OneToManyassociations or just the@ManyToOneside.