EclipseLink/Development/JPA/QueryDownCast

Enhancement: Query Down Cast

This page captures the requirements, design, and existing functionality for bug ? to enable EclipseLink JPA/ORM developers to define queries on inheritance hierarchies with down casting to specific classes. Requirements

Work-Arounds

Since this feature will take significant development effort the following suggest work-around options are provided to assist users who require this functionality. If having this is a show-stopper for your project please vote for and add your feedback to bug ?.

Single Class Results

While this may seem obvious it often important to point out all potential solutions. If the query you are executing will only be returning a single type from the inheritance hierarchy then it is important that that be the target type of the query. This will allow you to access all mapped attributes in this class and its mapped parent classes.

Accessing un-mapped attributes using QueryKey

When your inheritance hierarchy leverages a common table for multiple mapped classes in the hierarchy it is possible to query for attributes that are not visible in the class you are querying for through the use of query keys.

Example

In this example we'll map a simple inheritance hierarchy of class A having two subclasses B and C. Each class will have its own value and they will all be mapped to a single table.

@Entity
@Table(name="DOWNCAST_SIMPLE_A")
@Inheritance(strategy=InheritanceType.SINGLE_TABLE)
@DiscriminatorColumn(name="INH_TYPE",discriminatorType=DiscriminatorType.CHAR)
public abstract class A {
	@Id
	private int id;
 
	private String aValue;
 
	// accessor methods
}
 
@Entity
@DiscriminatorValue("B")
public class B  extends A{
 
	private String bValue;
 
	// accessor methods
}
 
@Entity
@DiscriminatorValue("C")
public class C extends A {
 
	private String cValue;
 
	// accessor methods
}

Based on this mapped entity model the generated schema looks like:

CREATE TABLE DOWNCAST_SIMPLE_A (
		ID NUMBER(10) NOT NULL, 
		INH_TYPE VARCHAR2(31) NULL, 
		AVALUE VARCHAR2(255) NULL, 
		BVALUE VARCHAR2(255) NULL, 
		CVALUE VARCHAR2(255) NULL,
	 PRIMARY KEY (ID))

Now to build a heterogenous query for A using bValue and cValue I need to define query keys on A to make this fields visible.

Solution Design

TBD