The Transient annotation specifies that a field is non-persistent. Use it to exclude fields from management that would otherwise be persistent. Transient is a marker annotation only; it has no properties.

The equivalent XML element is transient. It has a single attribute:

Annotate your simple identity fields with Id. This annotation has no properties. We explore entity identity and identity fields in Section 4.1.3, “Identity Fields”.

The equivalent XML element is id. It has one required attribute:

The previous section showed you how to declare your identity fields with the Id annotation. It is often convenient to allow the persistence implementation to assign a unique value to your identity fields automatically. JPA includes the the GeneratedValue annotation for this purpose. You can only apply the GeneratedValue annotation to numeric fields. It has the following properties:

The equivalent XML element is generated-value, which includes the following attributes:

Note

Kodo allows you to use the GeneratedValue annotation on any field, not just identity fields. Before using the IDENTITY generation strategy, however, read Section 5.3.3, “Autoassign / Identity Strategy Caveats” in the Reference Guide. Kodo also offers two additional generator strategies for non-numeric fields, which you can access by setting strategy to AUTO (the default), and setting the generator string to: uuid-string: Kodo will generate a 128-bit UUID unique within the network, represented as a 16-character string. For more information on UUIDs, see the IETF UUID draft specification at: http://www1.ics.uci.edu/~ejw/authoring/uuid-guid/ uuid-hex: Same as uuid-string, but represents the UUID as a 32-character hexadecimal string. These string constants are defined in kodo.persistence.Generator.

If your entity has multiple identity values, you may declare multiple @Id fields, or you may declare a single @EmbeddedId field. The type of a field annotated with EmbeddedId must be an embeddable entity class. The fields of this embeddable class are considered the identity values of the owning entity. We explore entity identity and identity fields in Section 4.1.3, “Identity Fields”.

The EmbeddedId annotation has no properties.

The equivalent XML element is embedded-id. It has one required attribute:

Use the Version annotation to designate a version field. Section 4.1.4, “Version Field” explained the importance of version fields to JPA. This is a marker annotation; it has no properties.

The equivalent XML element is version, which has a single attribute:

Basic signifies a standard value persisted as-is to the datastore. You can use the Basic annotation on persistent fields of the following types: primitives, primitive wrappers, java.lang.String, byte[], Byte[], char[], Character[], java.math.BigDecimal, java.math.BigInteger, java.util.Date, java.util.Calendar, java.sql.Date, java.sql.Timestamp, Enums, and Serializable types.

Basic declares these properties:

The equivalent XML element is basic. It has the following attributes:

5.2.6.1. Fetch Type

Many metadata annotations in JPA have a fetch property. This property can take on one of two values: FetchType.EAGER or FetchType.LAZY. FetchType.EAGER means that the field is loaded by the JPA implementation before it returns the persistent object to you. Whenever you retrieve an entity from a query or from the EntityManager, you are guaranteed that all of its eager fields are populated with datastore data.

FetchType.LAZY is a hint to the JPA runtime that you want to defer loading of the field until you access it. This is called lazy loading. Lazy loading is completely transparent; when you attempt to read the field for the first time, the JPA runtime will load the value from the datastore and populate the field automatically. Lazy loading is only a hint and not a directive because some JPA implementations cannot lazy-load certain field types.

With a mix of eager and lazily-loaded fields, you can ensure that commonly-used fields load efficiently, and that other state loads transparently when accessed. As you will see in Section 7.3, “Persistence Context”, you can also use eager fetching to ensure that entites have all needed data loaded before they become detached at the end of a persistence context.

	Note
	Kodo can lazy-load any field type. Kodo also allows you to dynamically change which fields are eagerly or lazily loaded at runtime. See Section 5.6, “Fetch Groups” in the Reference Guide for details. The Reference Guide details Kodo's eager fetching behavior in Section 5.7, “Eager Fetching”.

Use the Embedded marker annotation on embeddable field types. Embedded fields are mapped as part of the datastore record of the declaring entity. In our sample model, Author and Company each embed their Address, rather than forming a relation to an Address as a separate entity.

The equivalent XML element is embedded, which expects a single attribute:

When an entity A references a single entity B, and other As might also reference the same B, we say there is a many-to-one relation from A to B. In our sample model, for example, each magazine has a reference to its publisher. Multiple magazines might have the same publisher. We say, then, that the Magazine.publisher field is a many-to-one relation from magazines to publishers.

JPA indicates many-to-one relations between entities with the ManyToOne annotation. This annotation has the following properties:

The equivalent XML element is many-to-one. It accepts the following attributes:

@ManyToOne(cascade={CascadeType.PERSIST,CascadeType.REMOVE,
    CascadeType.REFRESH,CascadeType.MERGE})
private Company publisher;

@ManyToOne(cascade=CascadeType.ALL)
private Company publisher;

<many-to-one name="publisher">
    <cascade>
        <cascade-persist/>
        <cascade-merge/>
        <cascade-remove/>
        <cascade-refresh/>
    </cascade>
</many-to-one>

<many-to-one name="publisher">
    <cascade>
        <cascade-all/>
    </cascade>
</many-to-one>

When an entity A references multiple B entities, and no two As reference the same B, we say there is a one-to-many relation from A to B.

One-to-many relations are the exact inverse of the many-to-one relations we detailed in the preceding section. In that section, we said that the Magazine.publisher field is a many-to-one relation from magazines to publishers. But now we see that the Company.mags field is the inverse - a one-to-many relation from publishers to magazines. Each company may publish multiple magazines, but each magazine can have only one publisher.

JPA indicates one-to-many relations between entities with the OneToMany annotation. This annotation has the following properties:

The equivalent XML element is one-to-many, which includes the following attributes:

You may also nest the cascade element within a one-to-many element.

5.2.9.1. Bidirectional Relations

When two fields are logical inverses of each other, they form a bidirectional relation. Our model contains two bidirectional relations: Magazine.publisher and Company.mags form one bidirectional relation, and Article.authors and Author.articles form the other. In both cases, there is a clear link between the two fields that form the relationship. A magazine refers to its publisher while the publisher refers to all its published magazines. An article refers to its authors while each author refers to his or her written articles.

When the two fields of a bidirectional relation share the same datastore mapping, JPA formalizes the connection with the mappedBy property. Marking Company.mags as mappedBy Magazine.publisher means two things:

1. Company.mags uses the datastore mapping for Magazine.publisher, but inverses it. In fact, it is illegal to specify any additional mapping information when you use the mappedBy property. All mapping information is read from the referenced field. We explore mapping in depth in Chapter 12, Mapping Metadata.

2. Magazine.publisher is the "owner" of the relation. The field that specifies the mapping data is always the owner. This means that changes to the Magazine.publisher field are reflected in the datastore, while changes to the Company.mags field alone are not. Changes to Company.mags may still affect the JPA implementation's cache, however. Thus, it is very important that you keep your object model consistent by properly maintaining both sides of your bidirectional relations at all times.

You should always take advantage of the mappedBy property rather than mapping each field of a bidirectional relation independently. Failing to do so may result in the JPA implementation trying to update the database with conflicting data. Be careful, however, to only mark one side of the relation as mappedBy. One side has to actually do the mapping!

	Note
	You can configure Kodo to automatically synchronize both sides of a bidirectional relation, or to perform various actions when it detects inconsistent relations. See Section 5.4, “Managed Inverses” in the Reference Guide for details.

When an entity A references a single entity B, and no other As can reference the same B, we say there is a one-to-one relation between A and B. In our sample model, Magazine has a one-to-one relation to Article through the Magazine.coverArticle field. No two magazines can have the same cover article.

JPA indicates one-to-one relations between entities with the OneToOne annotation. This annotation has the following properties:

The equivalent XML element is one-to-one, which understands the following attributes:

You may also nest the cascade element within a one-to-one element.

When an entity A references multiple B entities, and other As might reference some of the same Bs, we say there is a many-to-many relation between A and B. In our sample model, for example, each article has a reference to all the authors that contributed to the article. Other articles might have some of the same authors. We say, then, that Article and Author have a many-to-many relation through the Article.authors field.

JPA indicates many-to-many relations between entities with the ManyToMany annotation. This annotation has the following properties:

The equivalent XML element is many-to-many. It accepts the following attributes:

You may also nest the cascade element within a many-to-many element.

Datastores such as relational databases do not preserve the order of records. Your persistent List fields might be ordered one way the first time you retrieve an object from the datastore, and a completely different way the next. To ensure consistent ordering of collection fields, you must use the OrderBy annotation. The OrderBy annotation's value is a string defining the order of the collection elements. An empty value means to sort on the identity value(s) of the elements in ascending order. Any other value must be of the form:

<field name>[ ASC|DESC][, ...]

Each <field name> is the name of a persistent field in the collection's element type. You can optionally follow each field by the keyword ASC for ascending order, or DESC for descending order. If the direction is omitted, it defaults to ascending.

The equivalent XML element is order-by, which can be listed as a sub-element of the one-to-many or many-to-many elements. The text within this element is parsed as the order by string.

	Note
	Kodo expands the available ordering syntax. See Section 6.4.2.3, “Order-By” in the Reference Guide for details.

JPA supports persistent Map fields through either a OneToMany or ManyToMany association. The related entities form the map values. JPA derives the map keys by extracting a field from each entity value. The MapKey annotation designates the field that is used as the key. It has the following properties:

The equivalent XML element is map-key, which can be listed as a sub-element of the one-to-many or many-to-many elements. The map-key element has the following attributes:

In the absence of any of the annotations above, JPA defines the following default behavior for declared fields:

Note that according to these defaults, all relations between entities must be annotated explicitly. Without an annotation, a relation field will default to serialized storage if the related entity type is serializable, or will default to being non-persistent if not.