Difference between revisions of "EclipseLink/Examples/JPA/Migration/OpenJPA/Mappings"

Revision as of 12:59, 6 November 2014

OpenJPA to EclipseLink JPA Migration: Mappings

The following are some mapping scenarios encountered when migrating from OpenJPA to EclipseLink. If your scenario is not captured here please file a enhancement request referencing this page that describes the mapping related migration issue you are facing.

Mapping Assistance with Dali

In general it is recommended to use a JPA mapping tool such as Eclipse Dali (WTP) to map the entities. It is built into Eclipse 3.4 and higher within WTP and allows excellent validation and configuration assistance to address many common mapping errors.

@Column in Relationship Mappings

When defining relationship mappings involving Foriegn Keys (@OneToOne and @ManyToOne) JPA supports the specification of non-default column names using @JoinColumn and @JoinColumns. OpenJPA allows the specification of @Column. Since this has no defined meaning under JPA EclipseLink throws a validation exception to assist customers in identifying this incorrect mapping configuration.

Solution

Convert all @OneToOne and @ManyToOne mappings that have an @Column configuration to use @JoinColumn. If the target class of the relationship has a composite identifier (primary key) then @JoinColumns will be required.

Note: If the relationship defined by the @JoinColumn(s) involves columns which are also mapped as identifiers (primary key columns) then additional care will be required to ensure these are mapped with @Id and the duplicate mappings are properly mapped and managed in the entity class.

@Transient with Relationship Mappings

It is undefined in the JPA specification to define a mapping (@Basic, @OneToOne, @ManyToOne, @OneToMany, @ManyToMany, ...) and transient. While OpenJPA allows this, EclipseLink correctly throws and exception indicating the conflict in the configuration. Transient configuration on an attribute is intended to prevent default mappings from being applied when calculating the mappings for an entity. With default mappings unmapped attributes are assumed according to the specification and @Transient prevents these assumptions.

Solution

Determine which configuration is wanted. If the mapping wanted then remove @Transient. If the intention was to avoid the default mapping then either remove or comment out the mapping configuration.

@GeneratedValue/@GeneratedValue(‘generator=AUTO’)

When a primary key is annotated by @GeneratedValue with ‘generator=AUTO’ specified or no attributes specified, OpenJPA and EclipseLink create different tables to generate values for the primary keys. If there are existing entities with primary keys generated by OpenJPA, persisting new entities using EclipseLink will cause an error since EclipseLink will be looking for its own table to generate these ids.

Solution

EclipseLink can add new entities with existing ones by configuring the entities to use OpenJPA’s table to generate the values of their primary keys. By doing so, EclipseLink will use the last value listed in the OpenJPA table to generate any new keys.
This solution is implemented by replacing the @GeneratedValue or @GeneratedValue(generator=AUTO) with the following annotations:
@TableGenerator(name = "OPENJPA_SEQUENCE_TABLE", table = "OPENJPA_SEQUENCE_TABLE", pkColumnName = "ID", valueColumnName = "SEQUENCE_VALUE", pkColumnValue = "0")
@GeneratedValue(strategy = GenerationType.TABLE, generator = "OPENJPA_SEQUENCE_TABLE")