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Difference between revisions of "EclipseLink/Release/2.1.0/JPAQueryEnhancements"

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= FUNC =
 
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This shows examples on how to use EclipseLink to define queries on inheritance hierarchies with down casting to specific classes.
 
This shows examples on how to use EclipseLink to define queries on inheritance hierarchies with down casting to specific classes.
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The changes as listed above in bold.
 
The changes as listed above in bold.
 
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Revision as of 11:48, 11 June 2010

FUNC

bug 300512

Overview

A new JPQL keyword has been added to JPQL to support native database functions.

To call the DB function MyFunc(a, b, c) use FUNC('MyFunc', a, b, c)

Year function

YEAR()
SELECT FUNC('YEAR', whenRegistered) Y, COUNT(registration.id) C FROM registration GROUP BY Y
JPQL equivalent to Oracle's NVL function

EclipseLink's ExpressionOperator.ifNull will generate "NVL". How can this be used in JPQL and Criteria defined queries?

// on Oracle
SELECT FUNC('NVL', e.firstName, 'NoFirstName'), func('NVL', e.lastName, 'NoLastName') FROM Employee e
// on MySQL
SELECT FUNC('IFNULL', e.firstName, 'NoFirstName'), func('IFNULL', e.lastName, 'NoLastName') FROM Employee e
Spatial Functionality via JPQL
PostGIS Intersects (forum)
SELECT a FROM Asset a, Geography selectedGeography
WHERE selectedGeography.id = :id AND a.id IN :id_list
AND FUNC('ST_Intersects', a.geometry, selectedGeography.geometry) = 'TRUE'
Oracle Spatial
SELECT ss FROM SimpleSpatial ss WHERE FUNC('mdsys.sdo_relate', ss.jGeometry, :otherGeometry, :params) = 'TRUE' ORDER BY ss.id ASC


Functions in select and order by clause

// This fails:
SELECT CONCAT(e.firstName, e.lastName) FROM Employee e 
// This works:
SELECT CONCAT(e.firstName, e.lastName) FROM Employee e WHERE e.id > 0
ORDER BY boolean

Fields and tables, "from X x, Table(Y) y", "x.field(y)"

?

Object parameters and nested parameters "(:employee).id"

Saw several times users attempting something like:
SELECT e FROM Employee e WHERE e.address IN (SELECT a FROM Address…)

Query Casting

bug 259266

This shows examples on how to use EclipseLink to define queries on inheritance hierarchies with down casting to specific classes.

Overview

JPA/ORM developers will use this query feature to query across attributes in subclasses. This feature is available in JPQL, EclipseLink Expressions and Criteria API.

JPA2.0 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

JPQL is extended to cast in the FROM clause. The format of this will use the keyword "TREAT" and be part of the join clause. The following is an example:

select e from Employee e join TREAT(e.projects AS LargeProject) lp where lp.budget = value

Criteria API

JPA Criteria API already provides a casting operator. It is Expression.as(type).

As it is defined by JPA 2.0, Expression.as(type) does a simple cast that allows matching of types within the generics.

EclipseLink 2.1 extends criteria API to allow a cast using Expression.as(type). The as method has been 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<Employee, Project> join = empRoot.join("projects");
Path exp = ((Path)join.as(LargeProject.class)).get("budget");
cq1.where(qb1.equal(exp, new Integer(1000)) );

Calling a cast on a JOIN node will permanently alter that node. i.e. In the example above, after calling join.as(LargeProject.class), join will refer to a LargeProject.

EclipseLink 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
  • It is prudent to do a check for type in a query that does a cast.
    • The following select f from Foo f join cast(f.bars, BarSubclass) b where b.subclassAttribute = "value"
    • Should be written as: select f from Foo f join cast(f.bars, BarSubclass) b where type(b) = BarSubclass And b.subclassAttribute = "value" by users that wish to enforce the type.
    • EclipseLink will automatically append type information for cases where the cast results in a single type, but for classes in the middle of a hierarchy, no type information will not be appended to the SQL

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.as(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.