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Hibernate ORM 6.2 - Composite aggregate mappings

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Hibernate ORM version 6.2.0.Final is around the corner, and the following post tries to shed some light on one of the new features that comes with that release.

Composite SQL types

For a long time, SQL was fully relational and every data model had to be modeled as table with columns and constraints. Then came the object relational hype and the SQL:1999 standard introduced support for structured types. With the XML hype coming next, the SQL:2003 standard added support for XML and in the SQL:2016 standard JSON support first appeared.

Structured or unstructured, composite types have their use cases, so it was about time that Hibernate came up with an answer for these data types.

Values of these composite types provide access to their sub-parts and do not have identity like a row of a table. In the realm of ORM, embeddable types have the same properties, so it is only natural to allow mapping to these SQL types.

Struct mapping

Mapping an embeddable to a named SQL object type, also known as structured type, is now possible by annotating either the embeddable type or the @Embedded field/property with @Struct(name = "…​").

The hbm2ddl schema generation tool supports generating DDL to create and drop these structured types. Since the order of structured type attributes is important and has to match the expectations of Hibernate, users can compare the type definition until schema validation adds support for structured types as well.

Considering this simple example mapping:

@Embeddable
@Struct(name = "my_point")
public class Point {
        private int x;
        private int y;
}

will produce a schema similar to the following:

create type my_point as (
    x int not null,
    y int not null
)

The name and the nullability of the struct column can be refined through applying a @Column on the persistent attribute.

@Entity
public class PointHolder {
        @Id
        private int id;
        @Column(name = "the_point", nullable = false)
        private Point p;
}

which will emit a schema similar to this:

create table PointHolder as (
    id int not null primary key,
    the_point my_point not null
)

Since the order of structured type attributes is vital, it is important to understand how to control the expectations of Hibernate. There are essentially two possible ways to control the order:

  • Specifying the attributes member on the @Struct annotation to define the order

  • Using a Java record to implicitly specify the order through the canonical constructor

The first option is pretty simple. If the desired order is (y,x), then the Point embeddable type needs to declare that by using @Struct(name = "my_point", attributes = {"y", "x"}).

The second option is more interesting as it makes use of another new feature that was introduced in Hibernate 6.2, which is the support for Java records out of the box.

@Embeddable
@Struct(name = "my_point")
public record Point(int y, int x) {}

There is no need for the @EmbeddableInstantiator anymore for Java records, because Hibernate now configures an appropriate instantiator automatically based on the canonical constructor. The order of the record components in that canonical constructor then also represents the order in which Hibernate expects the structured types attributes to be defined in.

At the time of writing, structured type support is only implemented for Oracle, PostgreSQL and DB2.

XML and JSON mapping

Since XML and JSON are predefined unstructured types, there is no type name or attribute order that has to be defined. Mapping an embeddable as XML or JSON can be done by annotating the @Embedded field/property with @JdbcTypeCode(SqlTypes.SQLXML) or @JdbcTypeCode(SqlTypes.JSON) respectively.

At the time of writing, there is no database dialect for which XML support has been implemented, so the following will focus on the JSON parts only, but apply the same way to XML support in the future. 
@Entity
public class JsonHolder {
        @Id
        private int id;
        @JdbcTypeCode(SqlTypes.JSON)
        @Column(name = "my_json", nullable = false)
        private Point point;
}

An entity with a mapping like this will roughly produce a schema like the following:

create table JsonHolder as (
    id int not null primary key,
    my_json json not null,
    check (json_value(my_json, '$.x') is not null and json_value(my_json, '$.y') is not null)
)

As you can see, the table definition contains a check constraint that enforces the constraints of the embeddable type within the JSON value by accessing the fields through the json_value function.

HQL queries that make use of embeddable attributes

select j.point.x, j.point.y
from JsonHolder j

will resolve to the respective json_value expression:

select json_value(j.point, '$.x'), json_value(j.point, '$.y')
from JsonHolder j

Similarly, assignment expressions will update the relevant part of the json document.

At the time of writing, JSON support is only implemented for Oracle and PostgreSQL.

Outlook

In upcoming releases we plan to add support for struct types on SQL Server, as well as support for other databases for the XML and JSON mappings. We will also try to lift restrictions on composite type mappings and enable the use of association mappings as well as array mappings.

I hope you like the new features and can provide us feedback about this in the forum or on our chat platform!

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