Best way to map the JPA and Hibernate ManyToMany relationship

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Introduction

In this article, I’m going to show you the best way to map a ManyToMany association when using JPA and Hibernate.

As simple as JPA annotations might be, it’s not always obvious how efficient they are behind the scenes. In this article, I’m going to show you what is the best way to use the JPA @ManyToMany annotation when using Hibernate.

Domain Model

Assuming we have the following database tables:

The post and tag many-to-many table relationship

A typical many-to-many database association includes two parent tables which are linked through a third one containing two Foreign Keys referencing the parent tables.

Implementing the ManyToMany JPA and Hibernate association using a List

The first choice for many Java developers is to use a java.util.List for Collections that don’t entail any specific ordering.

@Entity(name = "Post")
@Table(name = "post")
public class Post {

    @Id
    @GeneratedValue
    private Long id;

    private String title;

    public Post() {}

    public Post(String title) {
        this.title = title;
    }

    @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<>();

    //Getters and setters ommitted for brevity

    public void addTag(Tag tag) {
        tags.add(tag);
        tag.getPosts().add(this);
    }

    public void removeTag(Tag tag) {
        tags.remove(tag);
        tag.getPosts().remove(this);
    }

    @Override
    public boolean equals(Object o) {
        if (this == o) return true;
        if (!(o instanceof Post)) return false;
        return id != null && id.equals(((Post) o).getId());
    }

    @Override
    public int hashCode() {
        return 31;
    }
}

@Entity(name = "Tag")
@Table(name = "tag")
public class Tag {

    @Id
    @GeneratedValue
    private Long id;

    @NaturalId
    private String name;

    @ManyToMany(mappedBy = "tags")
    private List<Post> posts = new ArrayList<>();

    public Tag() {}

    public Tag(String name) {
        this.name = name;
    }

    //Getters and setters ommitted for brevity

    @Override
    public boolean equals(Object o) {
        if (this == o) return true;
        if (o == null || getClass() != o.getClass()) return false;
        Tag tag = (Tag) o;
        return Objects.equals(name, tag.name);
    }

    @Override
    public int hashCode() {
        return Objects.hash(name);
    }
} 

There are several aspects to note on the aforementioned mapping that are worth explaining/

First of all, the tags association in the Post entity only defines the PERSIST and MERGE cascade types. As explained in this article, the REMOVE entity state transition doesn’t make any sense for a @ManyToMany JPA association since it could trigger a chain deletion that would ultimately wipe both sides of the association.

As explained in this article, the add/remove utility methods are mandatory if you use bidirectional associations so that you can make sure that both sides of the association are in sync.

The Post entity uses the entity identifier for equality since it lacks any unique business key. As explained in this article, you can use the entity identifier for equality as long as you make sure that it stays consistent across all entity state transitions.

The Tag entity has a unique business key which is marked with the Hibernate-specific @NaturalId annotation. When that’s the case, the unique business key is the best candidate for equality checks.

The mappedBy attribute of the posts association in the Tag entity marks that, in this bidirectional relationship, the Post entity owns the association. This is needed since only one side can own a relationship, and changes are only propagated to the database from this particular side.

For more details about the @NaturalId annotation, check out this article.

Although the mapping is correct from a JPA and Hibernate perspective, from a database perspective, the previous ManyToMany relationship mapping is not efficient at all. To understand why it is so, you need to log and analyze the automated generated SQL statements.

Considering we have the following entities:

final Long postId = doInJPA(entityManager -> {
    Post post1 = new Post("JPA with Hibernate");
    Post post2 = new Post("Native Hibernate");

    Tag tag1 = new Tag("Java");
    Tag tag2 = new Tag("Hibernate");

    post1.addTag(tag1);
    post1.addTag(tag2);

    post2.addTag(tag1);

    entityManager.persist(post1);
    entityManager.persist(post2);

    return post1.id;
});

When removing a Tag entity from a Post:

doInJPA(entityManager -> {
    Tag tag1 = new Tag("Java");
    Post post1 = entityManager.find(Post.class, postId);
    post1.removeTag(tag1);
});

Hibernate generates the following SQL statements:

SELECT p.id AS id1_0_0_,
       t.id AS id1_2_1_,
       p.title AS title2_0_0_,
       t.name AS name2_2_1_,
       pt.post_id AS post_id1_1_0__,
       pt.tag_id AS tag_id2_1_0__
FROM   post p
INNER JOIN 
       post_tag pt 
ON     p.id = pt.post_id
INNER JOIN 
       tag t 
ON     pt.tag_id = t.id
WHERE  p.id = 1

DELETE FROM post_tag
WHERE  post_id = 1

INSERT INTO post_tag
       ( post_id, tag_id )
VALUES ( 1, 3 )

So, instead of deleting just one post_tag entry, Hibernate removes all post_tag rows associated to the given post_id and reinserts the remaining ones back afterward. This is not efficient at all because it’s extra work for the database, especially for recreating indexes associated with the underlying Foreign Keys.

For this reason, it’s not a good idea to use the java.util.List for @ManyToMany JPA associations.

Implementing the ManyToMany JPA and Hibernate association using a Set

Instead of a List, we can use a Set.

The Post entity tags association will be changed as follows:

@ManyToMany(cascade = { 
    CascadeType.PERSIST, 
    CascadeType.MERGE
})
@JoinTable(name = "post_tag",
    joinColumns = @JoinColumn(name = "post_id"),
    inverseJoinColumns = @JoinColumn(name = "tag_id")
)
private Set<Tag> tags = new HashSet<>();

And the Tag entity will undergo the same modification:

@ManyToMany(mappedBy = "tags")
private Set<Post> posts = new HashSet<>();

If you worry about the lack of a predefined entry order, then you can use either the @OrderBy or @OrderColumn JPA annotations.

@OrderBy does the sorting in-memory, after the entries are fetched from the database while @OrderColumn materializes the element order in a dedicated column that is stored in the post_tag link table.

Now, when rerunning the previous test case, Hibernate generates the following SQL statements:

SELECT p.id AS id1_0_0_,
       t.id AS id1_2_1_,
       p.title AS title2_0_0_,
       t.name AS name2_2_1_,
       pt.post_id AS post_id1_1_0__,
       pt.tag_id AS tag_id2_1_0__
FROM   post p
INNER JOIN 
       post_tag pt 
ON     p.id = pt.post_id
INNER JOIN 
       tag t 
ON     pt.tag_id = t.id
WHERE  p.id = 1

DELETE FROM post_tag
WHERE  post_id = 1 AND tag_id = 3

Much better! There is only one DELETE statement executed which removes the associated post_tag entry.

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Conclusion

Using JPA and Hibernate is very convenient since it can boost developer productivity. However, this does not mean that you have to sacrifice application performance.

By choosing the right mappings and data access pattern, you can make the difference between an application that barely crawls and one that runs at warp speed.

So, when using the @ManyToMany annotation, always use a java.util.Set and avoid the java.util.List.

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12 Comments on “Best way to map the JPA and Hibernate ManyToMany relationship

  1. Hello Vlad,
    Thank you for these articles.
    My many to many association can have collections with thousands of elements.
    So requests sometimes can take minutes to process. How can I optimize this?

    Thanks

    • It’s very simple. You remove the collection and replace it with a paginated JPQL query.

  2. It’s a good practice to initialize set fields(like posts) or it should be done in the getters ?
    By the way nice article

    • Initializing the fields is a good better idea for collections or non-nullable fields that need a default value.

  3. Hello Vlad,
    I have question about persisting Post with Tags, when @EnableTransactionManagement is enabled: to be honest, it doesn’t work.

    When I manage transactions directly in my DAOs, everything works fine: Post table, Tag table, and Post_Tag table have their records. But with @EnableTransactionManagement ( and with transaction stuff removed from DAOs), Hibernate only save in Post Table, Entity Table, but not Post_Tag table.

    I checked logs, and the transaction is run.

    Is there some magic property to set or something?

    • The EnableTransactionManagement is a Spring annotation, so if you are looking for something magical to fix your issue, you need to check the Spring documentation or ask this question on the Spring forum.

  4. Hello Vlad,

    Thank you for making this awesome blog. I’m learning a lot. I have a question. My project has a User who has a need for two sets. It’s a small app for rating beers. The user has a Set for BeersDrank and a Set for WishListedBeers. I’m new to Database management, but I’m thinking, these should be two Many-to-Many tables (between User and Beer classes). This would also imply, that I need two sets of AddBeer and RemoveBeer (add/remove BeerDrank and add/remove BeerWisher). Does this sound ok to you? Your input would be invaluable. Thank you in advance.

    • Yes, sounds ok. Two many-to-many associations would do it. The synchronization methods are only needed for bidirectional associations.

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