Skip to main content

Notice: This Wiki is now read only and edits are no longer possible. Please see: https://gitlab.eclipse.org/eclipsefdn/helpdesk/-/wikis/Wiki-shutdown-plan for the plan.

Jump to: navigation, search

Difference between revisions of "EclipseLink/Development/2.1/AdvancedJPA Queries/DownCast"

Line 35: Line 35:
  
 
* An exception will be thrown a query execution time if the class that is downcast to is not a subclass of the class of the query key being downcast.
 
* An exception will be thrown a query execution time if the class that is downcast to is not a subclass of the class of the query key being downcast.
 +
* Downcasts are only allowed on query keys. The parent-expression of downcast must be a QueryKeyExpression. 
 
* Using Expression.downCast(Class) will cause a set of joins that link the tables required to get the data for the subclass.  Note: This will limit results for the query and therefore using downcasts to several classes in the same query will return results based on those joins. (e.g. If LargeProject and MediumProject are both multiple-table-subclasses of Project that have their own table, downcasting to both LargeProject and MediumProject in the same query could result in no results being returned due to the joins)
 
* Using Expression.downCast(Class) will cause a set of joins that link the tables required to get the data for the subclass.  Note: This will limit results for the query and therefore using downcasts to several classes in the same query will return results based on those joins. (e.g. If LargeProject and MediumProject are both multiple-table-subclasses of Project that have their own table, downcasting to both LargeProject and MediumProject in the same query could result in no results being returned due to the joins)
  

Revision as of 13:25, 18 March 2010

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.

Expression Support for Downcast

We will implement Expression.downcast(Class). The following is an example of how one could use it:

       ReadAllQuery raq = new ReadAllQuery(Employee.class);
       Expression criteria = raq.getExpressionBuilder().anyOf("projects").downcast(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 a query execution time if the class that is downcast to is not a subclass of the class of the query key being downcast.
  • Downcasts are only allowed on query keys. The parent-expression of downcast must be a QueryKeyExpression.
  • Using Expression.downCast(Class) will cause a set of joins that link the tables required to get the data for the subclass. Note: This will limit results for the query and therefore using downcasts to several classes in the same query will return results based on those joins. (e.g. If LargeProject and MediumProject are both multiple-table-subclasses of Project that have their own table, downcasting to both LargeProject and MediumProject in the same query could result in no results being returned due to the joins)


JPQL Extensions to use Downcast

This part of the feature will be considered only after the Expression support is complete.


Open Issues

  • The initial implementation will only allow downcasts in the WHERE clause. Unless there is feedback that indicates this is a major limitation, that will remain true in the final implementation.
  • Downcasts for table-per-class-inheritance still require some investigation and may be disallowed depending on the results of the investigation

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

Back to the top