JPA Multiple Embedded fields

Is it possible for a JPA entity class to contain two embedded (@Embedded) fields? An example would be:

@Entity
public class Person {
    @Embedded
    public Address home;

    @Embedded
    public Address work;
}

public class Address {
    public String street;
    ...
}

In this case a Person can contain two Address instances - home and work. I'm using JPA with Hibernate's implementation. When I generate the schema using Hibernate Tools, it only embeds one Address. What I'd like is two embedded Address instances, each with its column names distinguished or pre-pended with some prefix (such as home and work). I know of @AttributeOverrides, but this requires that each attribute be individually overridden. This can get cumbersome if the embedded object (Address) gets big as each column needs to be individually overridden.

Answers


If you want to have the same embeddable object type twice in the same entity, the column name defaulting will not work: at least one of the columns will have to be explicit. Hibernate goes beyond the EJB3 spec and allows you to enhance the defaulting mechanism through the NamingStrategy. DefaultComponentSafeNamingStrategy is a small improvement over the default EJB3NamingStrategy that allows embedded objects to be defaulted even if used twice in the same entity.

From Hibernate Annotations Doc: http://docs.jboss.org/hibernate/stable/annotations/reference/en/html_single/#d0e714


The generic JPA way to do it is with @AttributeOverride. This should work in both EclipseLink and Hibernate.

@Entity 
public class Person {
  @AttributeOverrides({
    @AttributeOverride(name="street",column=@Column(name="homeStreet")),
    ...
  })
  @Embedded public Address home;

  @AttributeOverrides({
    @AttributeOverride(name="street",column=@Column(name="workStreet")),
    ...
  })
  @Embedded public Address work;
  }

  @Embeddable public class Address {
    @Basic public String street;
    ...
  }
}

When using Eclipse Link, an alternative to using AttributeOverrides it to use a SessionCustomizer. This solves the issue for all entities in one go:

public class EmbeddedFieldNamesSessionCustomizer implements SessionCustomizer {

@SuppressWarnings("rawtypes")
@Override
public void customize(Session session) throws Exception {
    Map<Class, ClassDescriptor> descriptors = session.getDescriptors();
    for (ClassDescriptor classDescriptor : descriptors.values()) {
        for (DatabaseMapping databaseMapping : classDescriptor.getMappings()) {
            if (databaseMapping.isAggregateObjectMapping()) {
                AggregateObjectMapping m = (AggregateObjectMapping) databaseMapping;
                Map<String, DatabaseField> mapping = m.getAggregateToSourceFields();

                ClassDescriptor refDesc = descriptors.get(m.getReferenceClass());
                for (DatabaseMapping refMapping : refDesc.getMappings()) {
                    if (refMapping.isDirectToFieldMapping()) {
                        DirectToFieldMapping refDirectMapping = (DirectToFieldMapping) refMapping;
                        String refFieldName = refDirectMapping.getField().getName();
                        if (!mapping.containsKey(refFieldName)) {
                            DatabaseField mappedField = refDirectMapping.getField().clone();
                            mappedField.setName(m.getAttributeName() + "_" + mappedField.getName());
                            mapping.put(refFieldName, mappedField);
                        }
                    }

                }
            }

        }
    }
}

}

In case you are using hibernate you can also use a different naming scheme which adds unique prefixes to columns for identical embedded fields. See Automatically Add a Prefix to Column Names for @Embeddable Classes


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