Why @OneToMany Does not Work with Inheritance in Hibernate?

Last Updated : 11 Oct, 2024

Hibernate, a popular ORM (Object-Relational Mapping) framework, helps map Java classes to database tables. While Hibernate supports @OneToMany relationships, developers may face issues when inheritance is introduced in these relationships. This article will explore why @OneToMany relationships do not work smoothly with inheritance and what solutions can be applied.

In Hibernate, the @OneToMany annotation defines one-to-many relationships between entities. A simple use case of @OneToMany works as expected, but the problem arises when inheritance is involved.

Let's start by looking at a simple scenario.

Example: Author and Book Relationship

Author.java

@Entity
public class Author {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;  // Primary key

    // One-to-many relationship to Book, mapped by 'author' field in Book
    @OneToMany(mappedBy = "author")
    private List<Book> books; 

    // Getters and setters
}

The Author entity has a one-to-many relationship with Book.
The @OneToMany annotation is used to define this relationship, and the mappedBy attribute refers to the author field in the Book class.

Book.java

@Entity
public class Book {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;  // Primary key

    // Many-to-one relationship to Author
    @ManyToOne
    @JoinColumn(name = "author_id")  // Join column for author foreign key
    private Author author;  

    // Getters and setters
}

The Book class has a many-to-one relationship with Author.
The @ManyToOne annotation defines the relationship, and @JoinColumn sets up the foreign key column (author_id) in the Book table.

In this example, the @OneToMany relationship between Author and Book works without any problems because there’s no inheritance.

Now, let's introduce inheritance and see where issues arise.

Inheritance in Hibernate

Hibernate supports different inheritance strategies for mapping Java classes to database tables:

Single Table (SINGLE_TABLE): All classes in the hierarchy are stored in one table.
Joined Table (JOINED): Each class has its own table.
Table per Class (TABLE_PER_CLASS): Each class has its own table, without any inheritance relationship between them.

Scenario with Inheritance

Let’s introduce an inheritance hierarchy for Publication, where Book and Magazine are subclasses.

Publication.java

@Entity
@Inheritance(strategy = InheritanceType.JOINED)  // Use JOINED strategy for inheritance
public class Publication {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;  // Primary key for Publication

    // Common properties for all publications
    private String title;

    // Getters and setters
}

Publication is the base class.
The @Inheritance annotation with the JOINED strategy tells Hibernate to map subclasses to separate tables and link them to the parent table.

Book.java

@Entity
public class Book extends Publication {
    // Book-specific properties
    private String isbn;

    // Getters and setters
}

Book inherits from Publication and has additional properties (isbn).
The inheritance is handled by Hibernate, but no specific annotations are needed for Book since it inherits from Publication.

Magazine.java

@Entity
public class Magazine extends Publication {
    // Magazine-specific properties
    private String issueNumber;

    // Getters and setters
}

Magazine is another subclass of Publication, and it adds its own properties (issueNumber).
Just like Book, Magazine is part of the inheritance hierarchy.

Now, let's define the Library entity that contains a collection of Publication (including both Book and Magazine).

Library.java:

@Entity
public class Library {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;  // Primary key for Library

    // One-to-many relationship to Publication, which is a parent class in the hierarchy
    @OneToMany
    private List<Publication> publications;

    // Getters and setters
}

Library contains a list of Publication, which could hold either Book or Magazine.
The @OneToMany annotation is used to define this relationship with Publication.

Problem with `@OneToMany` and Inheritance

When we try to use a @OneToMany relationship with polymorphic entities (like Publication, Book, and Magazine), Hibernate has trouble handling the polymorphic nature of the collection.

Polymorphic Collections: Hibernate cannot determine which subclass of Publication (Book or Magazine) to load when retrieving Library.publications. This leads to errors during data retrieval.
Lazy Loading: When lazy-loading is used with a polymorphic collection, Hibernate might struggle to create proxies for the child classes, resulting in incomplete data.
Cascade Operations: Cascading operations such as PERSIST or REMOVE may not propagate correctly in a polymorphic collection, leading to data inconsistency.

Why Does This Happen?

The primary reason is that Hibernate expects collections to map to a single table. With polymorphism, different subclasses are stored in different tables, which complicates the query mechanism. Hibernate cannot easily infer which subclass instances to return from the Library.publications collection.

Solutions and Workarounds

To resolve these issues, several strategies can be employed.

1. Use `@MappedSuperclass` for Common Parent Class

If we do not need to persist instances of the parent class (Publication) directly, use @MappedSuperclass. This way, Publication won’t be treated as an entity, and you can avoid the complications of inheritance.

@MappedSuperclass
public abstract class Publication {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;  // Common ID for all subclasses

    private String title;  // Common title field

    // Getters and setters
}

With @MappedSuperclass, Publication becomes an abstract class and won’t have its own table.
Only Book and Magazine will have their own tables, simplifying the @OneToMany mapping.

2. Use Explicit Queries to Handle Polymorphic Collections

Instead of relying on Hibernate to automatically handle polymorphism, you can define specific queries for each subclass. For example, retrieving all Books in a Library:

SELECT b FROM Book b WHERE b.library.id = :libraryId

By explicitly querying for subclasses (Book, Magazine), you take control of the query mechanism and avoid the pitfalls of polymorphic collections.

3. Avoid Polymorphic Collections

We can split the polymorphic collection into multiple homogeneous collections. For example:

For example:

@Entity
public class Library {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;  // Primary key

    // Separate collections for Books and Magazines
    @OneToMany
    private List<Book> books;

    @OneToMany
    private List<Magazine> magazines;

    // Getters and setters
}

This approach separates Book and Magazine collections, which eliminates the polymorphism problem and simplifies the relationship management.

4. Use SINGLE_TABLE Inheritance with `@DiscriminatorColumn`

We can simplify inheritance mapping by using the SINGLE_TABLE strategy and adding a discriminator column to distinguish between subclasses.

@Entity
@Inheritance(strategy = InheritanceType.SINGLE_TABLE)
@DiscriminatorColumn(name = "publication_type")  // Column to distinguish subclass type
public class Publication {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;  // Common ID for all types of publications

    private String title;

    // Getters and setters
}

The SINGLE_TABLE strategy stores all entities in one table with a publication_type column to identify the subclass.
This approach avoids the need for joins or multiple tables, simplifying the @OneToMany relationship

Conclusion

The issues with @OneToMany and inheritance in Hibernate stem from the complexities of handling polymorphic collections across multiple tables. Hibernate struggles to manage relationships when inheritance is involved, but by applying workarounds like @MappedSuperclass, explicit queries, or inheritance strategies like SINGLE_TABLE, you can resolve these problems effectively.

By carefully choosing the mapping strategy and relationship design, you can avoid the challenges associated with @OneToMany and inheritance in Hibernate.