Introduction

One of my Twitter followers has recently asked me about a way of storing EAV (Entity-Attribute-Value) data using JPA and Hibernate, and, because this is a very good question, I decided to turn into a blog post.

Kudos to @vlad_mihalcea, the Hibernate Types library saved me tons of work yesterday by automagically mapping the PostgreSQL jsonb column to a POJO. Brilliant work 💪

— Tomasz Knyziak (@TomaszKnyziak) September 18, 2019

In this article, you are going to see how you can use JSON types to store schema-less EAV data.

Domain Model

Let’s assume we have an online book store, and books are modeled via the Book entity:

@Entity(name = "Book")
@Table(name = "book")
@TypeDef(
    typeClass = JsonType.class, 
    defaultForType = JsonNode.class
)
public class Book {

    @Id
    @GeneratedValue
    private Long id;

    @NaturalId
    private String isbn;

    @Column(columnDefinition = "jsonb")
    private JsonNode properties;

    //Getters and setters omitted for brevity
}

The Book has a unique isbn attribute, hence it’s mapped as a @NaturalId.

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

The properties attribute is of type JsonNode which contains the EAV data structure.

Because Hibernate does not support JSON types natively, we need a JsonType to handle the Jackson-specific JsonNode object type.

Hibernate Types

The first thing you need to do is to set up the following Maven dependency in your project pom.xml configuration file:

<dependency>
    <groupId>com.vladmihalcea</groupId>
    <artifactId>hibernate-types-55</artifactId>
    <version>${hibernate-types.version}</version>
</dependency>

That’s it!

Testing time

When persisting the following Book entity:

Book book = new Book();
book.setIsbn( "978-9730228236" );
book.setProperties(
    JacksonUtil.toJsonNode(
        "{" +
        "   \"title\": \"High-Performance Java Persistence\"," +
        "   \"author\": \"Vlad Mihalcea\"," +
        "   \"publisher\": \"Amazon\"," +
        "   \"price\": 44.99" +
        "}"
    )
);

entityManager.persist( book );

Hibernate generates the proper SQL INSERT statement:

INSERT INTO 
    book 
(
    isbn, 
    properties, 
    id
) 
VALUES 
(
    '978-9730228236', 
    '{"title":"High-Performance Java Persistence","author":"Vlad Mihalcea","publisher":"Amazon","price":44.99}',  
    1
)

We can also fetch the Book entity and even modify it, and Hibernate will take care of all the persistence logic:

Session session = entityManager.unwrap( Session.class );

Book book = session
    .bySimpleNaturalId( Book.class )
    .load( "978-9730228236" );

LOGGER.info( "Book details: {}", book.getProperties() );

book.setProperties(
    JacksonUtil.toJsonNode(
        "{" +
        "   \"title\": \"High-Performance Java Persistence\"," +
        "   \"author\": \"Vlad Mihalcea\"," +
        "   \"publisher\": \"Amazon\"," +
        "   \"price\": 44.99," +
        "   \"url\": \"https://www.amazon.com/High-Performance-Java-Persistence-Vlad-Mihalcea/dp/973022823X/\"" +
        "}"
    )
);

When running the test case above, Hibernate generates the following SQL statements:

SELECT  b.id AS id1_0_
FROM    book b
WHERE   b.isbn = '978-9730228236'

SELECT  b.id AS id1_0_0_ ,
        b.isbn AS isbn2_0_0_ ,
        b.properties AS properti3_0_0_
FROM    book b
WHERE   b.id = 1

-- Book details: {"price":44.99,"title":"High-Performance Java Persistence","author":"Vlad Mihalcea","publisher":"Amazon"}

UPDATE 
    book 
SET 
    properties = '{"title":"High-Performance Java Persistence","author":"Vlad Mihalcea","publisher":"Amazon","price":44.99,"url":"https://www.amazon.com/High-Performance-Java-Persistence-Vlad-Mihalcea/dp/973022823X/"}' 
WHERE 
    id = 1

Cool, right?

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Conclusion

Hibernate types are great! Even if Hibernate comes with a great variety of types, even Java 1.8 Date/Time, you can easily map custom types like ARRAY or JSON.

For JSON, not only that you can map JSON structures as Java objects as we saw in a previous article, but you benefit from schema-less structures as well.

Insert details about how the information is going to be processed

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