EclipseLink/Development/2.1/AdvancedJPA Queries/DownCast

Enhancement: Query Down Cast

bug 259266

This page captures the requirements, design, and existing functionality for bug 259266 to enable EclipseLink JPA/ORM developers to define queries on inheritance hierarchies with down casting to specific classes. Requirements Must Have Support extensions to the expression framework to cast to a specific subclass or limit the results to those of a specific subclass. Support specifying queries that use attributes only available in a subclass when querying on or through a relationship to a parent class in the persistent hierarchy. Nice to Have JPQL Extensions to support 'casting' when querying through relationships High Level Design Limiting results by type Extensions to the expression framework to limit the results to those of a specific subclass have already been implemented as part of the JPA 2.0 effort. Expression.type(Class) is available in the expression framework and equivalent functionality is available in JPQL. e.g. "select p from Employee e join e.projects p where type(p) = LargeProject" can be used to retrieve all the LargeProjects (Subclass of Project) from Employee. JPQL Extensions to use Downcast JQPL could be extended in two places to enable a cast. 1. In where WHERE clause 2. As part of a Join in the FROM clause WHERE Clause A WHERE Clause downcast would look something like the following (budget is an attribute only available on LargeProject): select e from Employee e join e.project p where TYPE(p) = LargeProject and CAST(p, LargeProject).budget = 1000 This option means that each call to cast will create a separate set of Table joins. i.e. if I wrote the following: select e from Employee e join e.project p where TYPE(p) = LargeProject and CAST(p, LargeProject).budget and CAST(p, LargeProject).budget2 = 2000 I would see two sets of joins to the table that represents LargeProject, one for CAST(p, LargeProject).budget and one for CAST(p, LargeProject).budget2. As a result, the preference is to do casting as Part of the FROM clause. From Clause There are several ways a FROM clause cast can be written With 2 aliases and a keyword e.g. select e from Employee e join e.projects p downcast to LargeProject lp where TYPE(lp) = LargeProject and lp.attribute = value With a single alias and a keyword select e from Employee e join e.projects downcast to LargeProject lp where TYPE(lp) = LargeProject and lp.attribute = value With a single alias and no keyword select e from Employee e join (LargeProject)e.projects lp where TYPE(lp) = LargeProject and lp.attribute = value The expression with 2 aliases makes it easier to write queries that use both the superclass and the subclass in their critiera. e.g. select e from Employee e join e.projects p downcast to LargeProject lp where lp.attribute = value or p.name = "MyProject" The single alias expressions, however, are more intuitive, can still cover those cases with syntax similar to the following: select e from Employee e join (LargeProject)e.projects lp, join e.projects p where TYPE(lp) = LargeProject and lp.attribute = value or p.name = "MyProject" I propose we use the alternative above with a single alias and no keyword. i.e. select e from Employee e join (LargeProject)e.projects lp where TYPE(lp) = LargeProject and lp.attribute = value Criteria API JPA Criteria API already provides a casting operator. It is Expression.as(type). At the moment, Expression.as(type) does a simple cast to allow generics to match up. We will extend criteria API to allow a cast using Expression.as(type). The as method will be extended to check the hierarchy and if type is a subclass of the type for the expression that as is being called on a cast will be implemented. Here is a criteria query that will do a cast: Root<Employee> empRoot = cq1.from(getEntityManagerFactory().getMetamodel().entity(Employee.class)); Join join = empRoot.join("projects"); Path exp = ((Path)join.as(LargeProject.class)).get("budget"); cq1.where(qb1.equal(exp, new Integer(1000)) ); Expression Support for Downcast We will implement Expression.as(Class). The following is an example of how one could use it: ReadAllQuery raq = new ReadAllQuery(Employee.class); Expression criteria = raq.getExpressionBuilder().anyOf("projects").as(LargeProject.class).get("budget").greaterThan(100); raq.setSelectionCriteria(criteria); List resultList = query.getResultList(); In this query Employee has a xToMany mapping to Project. LargeProject is a subclass of Project and the "budget" attribute is contained on LargeProject. An exception will be thrown at query execution time if the class that is cast to is not a subclass of the class of the query key being cast. Casts are only allowed on ObjectExpressions (QueryKeyExpression and ExpressionBuilder). The parent expression of a cast must be an ObjectExpression Casts use the same outer join settings as the ObjectExpression they modify Casts modify their parent expression. As a result, when using a cast with a parallel expression, you must use a new instance of the parent expression. Casting is not supported for TablePerClass Inheritance Cast expressions must be used in conjuction with type expressions to enforce the getting the correct type. The following select f from Foo f join f.bars b downcast as BarSubclass s where s.subclassAttribute = "value" Should be written as: select f from Foo f join f.bars b downcast as BarSubclass s where type(s) = BarSubclass And s.subclassAttribute = "value" by users that wish to enforce the type. Generally, the joins will be adequate to enforce the type, but in cases where multiple subclasses use exactly the same table set or outer joins, the TYPE expression may be required to guarantee the type. Design API We will add the following public API: Expression downcast(Class downcastClass) The parent expression for this must be an ObjectExpression (ExpressionBuilder or QueryKeyExpression) and an exception will be thrown if this is not the case when the downcast method is called. An exception will be thrown at query execution time if either downcastClass is not part of a mapped inheritance relationship or if the inheritance uses Table Per Class Inheritance If downcastClass is not either equal to the class described by the parent or a subclass of the class described by the parent, an exception will be thrown when the query is executed Downcast will modify the descriptor that is used by its parent expression. The change in descriptor will trigger the extra joins necessary for the downcast to work. Example SQL The following query: Select e from Employee e join e.projects project Will currently produce the following sql: SELECT <select list> FROM CMP3_EMPLOYEE t1 LEFT OUTER JOIN CMP3_DEPT t0 ON (t0.ID = t1.DEPT_ID), CMP3_EMP_PROJ t4, CMP3_PROJECT t3, CMP3_SALARY t2 WHERE ((t2.EMP_ID = t1.EMP_ID) AND ((t4.EMPLOYEES_EMP_ID = t1.EMP_ID) AND (t3.PROJ_ID = t4.projects_PROJ_ID))) If we augment a select criteria like the following Expression criteria = project.downcast(LargeProject.class).get("budget").greaterThan(100); raq.setSelectionCriteria(criteria); The following SQL will be produced: SELECT <select list> FROM CMP3_PROJECT t3 LEFT OUTER JOIN CMP3_LPROJECT t4 ON (t4.PROJ_ID = t3.PROJ_ID),CMP3_EMPLOYEE t1 LEFT OUTER JOIN CMP3_DEPT t0 ON (t0.ID = t1.DEPT_ID), CMP3_EMP_PROJ t5, CMP3_SALARY t2 WHERE (((t4.BUDGET > ?) AND (t2.EMP_ID = t1.EMP_ID)) AND ((t5.EMPLOYEES_EMP_ID = t1.EMP_ID) AND (t3.PROJ_ID = t5.projects_PROJ_ID))) bind => [100.0] The changes as listed above in bold.

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 259266.

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.

// Configure the use of a customizer on the entity class
@Customizer(ACustomizer.class)
public abstract class A {
 
 
// The customizer adds the direct query keys
public class ACustomizer implements DescriptorCustomizer {
 
	public void customize(ClassDescriptor descriptor) throws Exception {
		descriptor.addDirectQueryKey("bValue", "BVALUE");
		descriptor.addDirectMapping("cValue", "CVALUE");
	}
 
}

Now you can write your query:

ReadAllQuery raq = new ReadAllQuery(A.class);
ExpressionBuilder eb = raq.getExpressionBuilder();
raq.setSelectionCriteria(eb.get("aValue").like("A%").and(eb.get("bValue").like("bValue")).and(eb.get("cValue").like("CVALUE")));
 
// Wrap in JPA Query
Query query = JpaHelper.createQuery(raq, em);
 
// Execute Query
List<A> results = query.getResultList();

The resulting SQL appears as:

SELECT ID, INH_TYPE, AVALUE, CVALUE, BVALUE FROM DOWNCAST_SIMPLE_A WHERE (((AVALUE LIKE ?) AND (BVALUE LIKE ?)) AND (CVALUE LIKE ?))

Querying JOINED Hierarchies using Joining

It is possible to query joined hierarchies as well:

Query query = em.createQuery("Select a from JoinedA a, JoinedB b, JoinedC c WHERE (b.bValue LIKE 'B%' and b = a) OR (c.cValue LIKE 'C%' and c = a)");
// Execute Query
List<JoinedA> results = query.getResultList();

The result SQL is:

[EL Fine]: Connection(27978063)--SELECT DISTINCT t0.INH_TYPE FROM DOWNCAST_JOINED_C t4, DOWNCAST_JOINED_A t3, DOWNCAST_JOINED_A t2, DOWNCAST_JOINED_B t1, DOWNCAST_JOINED_A t0 WHERE ((((t1.BVALUE LIKE ?) AND (t2.ID = t0.ID)) OR ((t4.CVALUE LIKE ?) AND (t2.ID = t3.ID))) AND (((t1.ID = t0.ID) AND (t0.INH_TYPE = ?)) AND ((t4.ID = t3.ID) AND (t3.INH_TYPE = ?))))
	bind => [B%, C%, B, C]

The challenge here is that the joining limits the results incorrectly for the OR condition and fails in some test cases.

Querying JOINED Hierarchies using IN

Another solution is to use an IN operator on each subclass you are interested in against a single part PK:

Select a from JoinedA a WHERE 
                        a.id IN (SELECT b.id FROM JoinedB b WHERE b.bValue LIKE 'B%') 
                        OR 
                        a.id IN (SELECT c.id FROM JoinedC c WHERE c.cValue LIKE 'C%')

the result SQL for this scenario is:

[EL Fine]: Connection(14707008)--SELECT DISTINCT t0.INH_TYPE FROM DOWNCAST_JOINED_A t0 WHERE (t0.ID IN (SELECT t1.ID FROM DOWNCAST_JOINED_B t2, DOWNCAST_JOINED_A t1 WHERE ((t2.BVALUE LIKE ?) AND ((t2.ID = t1.ID) AND (t1.INH_TYPE = ?)))) OR t0.ID IN (SELECT t3.ID FROM DOWNCAST_JOINED_C t4, DOWNCAST_JOINED_A t3 WHERE ((t4.CVALUE LIKE ?) AND ((t4.ID = t3.ID) AND (t3.INH_TYPE = ?)))))
	bind => [B%, B, C%, C]
[EL Fine]: Connection(14707008)--SELECT t0.ID, t0.INH_TYPE, t0.AVALUE, t1.ID, t1.BVALUE FROM DOWNCAST_JOINED_A t0, DOWNCAST_JOINED_B t1 WHERE ((t0.ID IN (SELECT t2.ID FROM DOWNCAST_JOINED_B t3, DOWNCAST_JOINED_A t2 WHERE ((t3.BVALUE LIKE ?) AND ((t3.ID = t2.ID) AND (t2.INH_TYPE = ?)))) OR t0.ID IN (SELECT t4.ID FROM DOWNCAST_JOINED_C t5, DOWNCAST_JOINED_A t4 WHERE ((t5.CVALUE LIKE ?) AND ((t5.ID = t4.ID) AND (t4.INH_TYPE = ?))))) AND ((t1.ID = t0.ID) AND (t0.INH_TYPE = ?)))
	bind => [B%, B, C%, C, B]
[EL Fine]: Connection(14707008)--SELECT t0.ID, t0.INH_TYPE, t0.AVALUE, t1.ID, t1.CVALUE FROM DOWNCAST_JOINED_A t0, DOWNCAST_JOINED_C t1 WHERE ((t0.ID IN (SELECT t2.ID FROM DOWNCAST_JOINED_B t3, DOWNCAST_JOINED_A t2 WHERE ((t3.BVALUE LIKE ?) AND ((t3.ID = t2.ID) AND (t2.INH_TYPE = ?)))) OR t0.ID IN (SELECT t4.ID FROM DOWNCAST_JOINED_C t5, DOWNCAST_JOINED_A t4 WHERE ((t5.CVALUE LIKE ?) AND ((t5.ID = t4.ID) AND (t4.INH_TYPE = ?))))) AND ((t1.ID = t0.ID) AND (t0.INH_TYPE = ?)))
	bind => [B%, B, C%, C, C]

Solution Design

TBD