If you want to map a @ManyToMany association with JPA you'll have to use a join table to represent the association.
Hibernate will automatically create the join table with two foreign keys:
public class Post {
...
@ManyToMany
@JoinTable( name = "post_tag",
joinColumns = @JoinColumn(referencedColumnName = "id", name = "postId"),
inverseJoinColumns = @JoinColumn(referencedColumnName = "id", name = "tagId"))
public List<Tag> tags;
...
}
But what if you want to add extra columns in this join table ? Mapping this association can be tricky.
This project provides an example of spring boot application with unit tests with a solution to this problem.
The solution is to create an entity "manually" to represent the association:
@Entity(name="PostTagEntity")
@Table(name="post_tag")
public class PostTag implements Serializable {
@EmbeddedId
// Our two foreign keys
// Check the real class for the pk implementation
public PostTagId id;
// Our extra column !!
Instant createdDate;
...
}
We have added an extra column createdDate in the join table.
Note that we have overrided the default @Entity name.
We can't use the default generated entity name "post_tag" because this entity name is already used by the @JoinTable definition in the Post entity,
this would throw an exception like "SQL strings added more than once for: post_tag".
We must override the entity name and and set whatever else, here PostTagEntity.
Because the entity name is different than the table name, we must set the @Table to map explicitly this entity on this table.
Once this is done, we must create the join table entries manually and save them in the database:
PostTag postTag = new PostTag();
postTag.id = ... // The join table foreign keys
postTag.createdDate = Instance.now(); // out extra column
save(postTag); // Saved with spring repository.
That's all ! The next time you'll get your Post entity, its tags list will be automatically filled:
Post myPost = findById(...);
myPost.tags; // The tags list is automatically filled
...
For more details, please check the implementation and the unit tests.