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For bi-directional relationships, the target object's inverse relationship must be back to the parent of the embeddable. The <code>mappedBy</code> can either reference the embeddable attribute directly, or use the dot notation from the parent class. | For bi-directional relationships, the target object's inverse relationship must be back to the parent of the embeddable. The <code>mappedBy</code> can either reference the embeddable attribute directly, or use the dot notation from the parent class. | ||
Relationships to another object's embeddable are not allowed. | Relationships to another object's embeddable are not allowed. | ||
+ | |||
+ | There is no standard way for an embeddable to reference its parent entity. The easiest way to implement this is to use property access to set the embedded object on the entity and in the set method set the back reference from the embeddable to its parent. The parent would be marked as <code>@Transient</code> as it is not a persistent relationship. | ||
======''Example: Embeddable with relationships''====== | ======''Example: Embeddable with relationships''====== |
Revision as of 14:05, 10 August 2011
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Contents
@Embeddable
You can use the @Embeddable annotation or <embeddable>
XML element to map an embedded class. An embeddable is a special type of class that is not directly persistent, but persisted only with its parent entity. An embeddable has no persistent identity, and no Id mapping. An embeddable can contain any type of attribute mapping including @Basic, @OneToOne, @ManyToOne, @OneToMany, @ManyToMany, @ElementCollection or another @Embedded. Embeddables cannot contain an @Id mapping.
An embeddable can be used to define an entities Id, normally in the case of a composite id. The id attributes in the embeddable will be mapped as @Basic not as @Id, as all attributes are required to be part of the @Id. The parent entity will use the @EmbeddedId to map the embeddeable instead of the @Embedded mapping.
An embeddable can be referenced from an entity or another embeddable using the @Embedded annotation for a single reference, or the @ElementCollection annotation for a Collection or Map reference. An embeddable can also be used in any Map key using the @MapKeyClass annotation.
Embeddables do not currently support the @Inheritance annotation, but inheritance is supported in EclipseLink through usage of a DescriptorCustomizer.
An embeddable cannot define a @Table annotation, and the columns used in its mappings are be default the names of the column is its parent's table. If the embeddable class is shared with multiple parents, then each parent can override the column names using the @AttributeOverride and @AssociationOverride annotations.
The @Embeddable annotation does not have any attributes.
For more information, see Section 2.5 "Embeddable Classes" in the JPA Specification.
For more information, see Section 11.1.13 "Embeddable Annotation" in the JPA Specification.
The following example shows usage of an embbeddable.
Example: Using @Embeddable annotation
@Embeddable public class EmployeementStatus implements Serializable { @Basic private String status; @Basic private String level; @Basic private java.sql.Date startDate; @Basic private java.sql.Date endDate; ... }
@Entity @Table(name="EMP") public class Employee implements Serializable { @Id private Long id; @Basic private String name; @Embedded private EmployeementStatus status; ... }
Example: Using <embeddable>
XML
<entity class="Employee"> <table name="EMP"> <attributes> <id name="id"/> <basic name="name"/> <embeddable name="status"/> ... </attributes> </entity> <embeddable class="EmployeementStatus"> <attributes> <basic name="status"/> <basic name="level"/> <basic name="startDate"/> <basic name="endDate"/> </attributes> </embeddable >
Advanced Embeddable Configuration
Sharing
A embeddable class can be shared by more than one class, or even referenced twice in the same class. Since an embeddable does not define a table, this means the class will stored in multiple different tables. As long as all of the table that stored the embeddable have its columns this is fine. If any of the table have different columns names, this must be handle through using @AttributeOverride
and @AssociationOverride
.
Note that the same embeddable object instance should never be shared, each parent should have its own object instance as the objects are always stored in their parents table. Only the class can be shared.
@AttributeOverride
You can use the @AttributeOverride and @AttributeOverrides annotations, or <attribute-override>
XML element to override the column for a basic attribute in an embedded relationship mapping. This allows for the column name to be different if the embeddable class is used by more than entity or if an entity defines multiple embedded relationships.
Attribute | Description | Default | Required? |
---|---|---|---|
name | The name of the attribute. | Yes | |
column | The column in the source table for the embedded relationship. | column defined in embeddable |
No |
For more information, see Section 11.1.4 "AttributeOverride Annotation" in the JPA Specification.
@AssociationOverride
You can use the @AssociationOverride and @AssociationOverrides annotations, or <association-override>
XML element to override the join column or join table for a relationship attribute in an embedded relationship mapping. This allows for the join column name to be different if the embeddable class is used by more than entity or if an entity defines multiple embedded relationships.
Attribute | Description | Default | Required? |
---|---|---|---|
name | The name of the attribute. | Yes | |
joinColumn | The join column in the source table for the embedded relationship. | join column defined in embeddable |
No |
joinTable | The join table for the subclass. | join table defined in embeddable |
No |
For more information, see Section 11.1.2 "AssociationOverride Annotation" in the JPA Specification.
@Embeddable public class Status implements Serializable { @Basic private String status; @Basic private String level; @Basic private java.sql.Date startDate; @Basic private java.sql.Date endDate; ... }
@Entity @Table(name="EMP") public class Employee implements Serializable { @Id private Long id; @Basic private String name; @Embedded @AttributeOverrides({ @AttributeOverride(name="level", column=@Column(name="EMPLOYMENT_LEVEL")) @AttributeOverride(name="status", column=@Column(name="EMPLOYMENT_STATUS"))}) private Status status; ... }
@Entity @Table(name="PROJ") public class Project implements Serializable { @Id private Long id; @Basic private String name; @Embedded @AttributeOverrides({ @AttributeOverride(name="level", column=@Column(name="PROJECT_LEVEL")) @AttributeOverride(name="status", column=@Column(name="PROJECT_STATUS"))}) private Status status; ... }
Relationships
Embeddable objects can define relationships. Any relationship type can be used including a nested embedded relationship.
For bi-directional relationships, the target object's inverse relationship must be back to the parent of the embeddable. The mappedBy
can either reference the embeddable attribute directly, or use the dot notation from the parent class.
Relationships to another object's embeddable are not allowed.
There is no standard way for an embeddable to reference its parent entity. The easiest way to implement this is to use property access to set the embedded object on the entity and in the set method set the back reference from the embeddable to its parent. The parent would be marked as @Transient
as it is not a persistent relationship.
Example: Embeddable with relationships
@Embeddable public class ProjectInfo implements Serializable { @Basic private boolean available; @ManyToMany private List<Project> projects; ... }
@Entity @Table(name="EMP") public class Employee implements Serializable { @Id private Long id; @Basic private String name; @Embedded private ProjectInfo projectInfo; ... }
@Entity @Table(name="PROJ") public class Project implements Serializable { @Id private Long id; @Basic private String name; @ManyToMany(mappedBy="projectInfo.projects") private List<Employee> employees; ... }
Nesting
An embeddable object can be embedded inside another embeddable object. All of the columns will be aggregated into the parent entity's table.
If an entity needs to override the column names for a nested embeddable the dot notation can be used inside the @AttributeOverride
.
Nested embeddables cannot contain embedded references to the same embeddable class or cycles, as there would be no way to store this in a relational table.
Example: Nested embeddable
@Embeddable public class ProjectInfo implements Serializable { @Basic private String name; @Basic private BigDecimal budget; @Embedded private ProjectStatus status; ... }
@Embeddable public class ProjectStatus implements Serializable { @Basic private String status; @Basic private boolean isBehindSchedule; ... }
@Entity public class Project implements Serializable { @Id private long id; @Embedded @AttributeOverride(name="budget", column=@Column(name="PROJECT_BUDGET")) @AttributeOverride(name="status.isBehindSchedule", column=@Column(name="BEHIND_SCHEDULE")) private ProjectInfo info; ... }
Inheritance
Embedded Ids
See, @EmbeddedId.
Nullable embedded values
Querying
Embeddable objects cannot be queried directly, but they can be queried in the context of their parent. Typically it is best to select the parent, and access the embeddable from the parent. This will ensure the embeddable is registered with the persistence context. If the embeddable is selected in a query, the resulting objects will be detached, and changes will not be tracked.
Example: Querying an embeddable
SELECT employee.period FROM Employee employee WHERE employee.period.endDate = :param