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Difference between revisions of "EclipseLink/Examples/JPA/MappingSelectionCriteria"
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Revision as of 11:30, 9 November 2009
Normally in the database a relationship is defined through a foreign key, either from the source table to the target table (ManyToOne), or from the target table to the source table (OneToMany). However, sometimes a relationship is more complex, and involves a comparison or computation. The JPA specification only allows relationships to be defines using foreign keys (JoinColumn), but EclipseLink also provides an extended Expression criteria API to allow complex relationships to be defined.
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Contents
Example 1: Department.manager (1:1)
Consider a Department that has a "manager" relationship to Employee. Instead of having a foreign key to its manager, the Employee has a foreign key to its Department, and an additional field that defines the Employee's position. EclipseLink does not currently define an annotation to define an Expression relationship so a DescriptorCustomizer can be used.
@Entity @Customizer(org.acme.persistence.DepartmentCustomizer.class) public class Department { @Id private long id; @OneToOne private Employee manager; @OneToMany(mappedBy="department") private List<Employee> employees; ... }
@Entity public class Employee { @Id private long id; @ManyToOne @JoinColumn(name="DEPT_ID") private Department department; }
import org.eclipse.persistence.config.DescriptorCustomizer; import org.eclipse.persistence.descriptors.ClassDescriptor; import org.eclipse.persistence.expressions.*; import org.eclipse.persistence.mappings.OneToOneMapping; public class DepartmentCustomizer implements DescriptorCustomizer { public void customize(ClassDescriptor descriptor) { OneToOneMapping mapping = (OneToOneMapping)descriptor.getMappingForAttributeName("manager"); ExpressionBuilder builder = new ExpressionBuilder(); mapping.setSelectionCriteria(builder.getField("EMPLOYEE.DEPT_ID").equal(builder.getParameter("DEPARTMENT.DEPT_ID"). and(builder.getField("EMPLOYEE.POSITION").equal("MANAGER")); } }
Example 2: 'active == true' in 1:M
Domain Model
@Entity @Table(name = "FILT_A") @Customizer(ConfigureBsFilter.class) public class A implements Serializable { @Id @Column(name = "A_ID") private int id; /** * Collection of Active B's. The application is responsible for ensuring * that when committing a TXN that no inactive B instances exist in the list */ @OneToMany(mappedBy="a", cascade=CascadeType.ALL) private List<B> bs;
@Entity @Table(name = "FILT_B") public class B { @Id @Column(name = "B_ID") private int id; @ManyToOne(fetch = FetchType.LAZY) @JoinColumn(name = "A_ID") private A a; private boolean active; private String value;
Customize A.Bs Criteria
public class ConfigureBsFilter implements DescriptorCustomizer { public void customize(ClassDescriptor descriptor) throws Exception { OneToManyMapping mapping = (OneToManyMapping) descriptor .getMappingForAttributeName("bs"); ExpressionBuilder eb = new ExpressionBuilder(mapping .getReferenceClass()); Expression fkExp = eb.getField("A_ID").equal(eb.getParameter("A_ID")); Expression activeExp = eb.get("active").equal(true); mapping.setSelectionCriteria(fkExp.and(activeExp)); } }