java - JPA entity without id

Question

I have a database with the following structure:

CREATE TABLE entity (
    id SERIAL,
    name VARCHAR(255),
    PRIMARY KEY (id)
);

CREATE TABLE entity_property (
    entity_id SERIAL,
    name VARCHAR(255),
    value TEXT
);

When I try to create an EntityProperty class

@Entity
@Table(name="entity_property")
public class EntityProperty {

    private String name;
    private String value;

    @Column(name="name")
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }

    @Column(name="value", nullable=true, length=255)
    public String getValue() {
        return value;
    }
    public void setValue(String value) {
        this.value = value;
    }
}

I get the following exception:

org.hibernate.AnnotationException: No identifier specified for entity: package.EntityProperty

I know that JPA entities must have a primary key but I can't change the database schema due to reasons that are beyond my control. Is it possible to create JPA (Hibernate) entities that will work with database schema like this?

Answer 1

I guess your entity_property has a composite key (entity_id, name) where entity_id is a foreign key to entity. If so, you can map it as follows:

@Embeddable
public class EntityPropertyPK {
    @Column(name = "name")
    private String name;

    @ManyToOne
    @JoinColumn(name = "entity_id")
    private Entity entity;

    ...
}

@Entity 
@Table(name="entity_property") 
public class EntityProperty { 
    @EmbeddedId
    private EntityPropertyPK id;

    @Column(name = "value")
    private String value; 

    ...
}