com.smartgwt.client.docs

## Interface JpaHibernateRelations

-     public interface JpaHibernateRelations

  ### JPA & Hibernate Relations

  JPA and Hibernate allow relations to be declared between entities where there is no actual Java field for storing the ID of a related entity, even though such a column exists in the database. For example:

           @ManyToOne
           @JoinColumn(name="countryId", referencedColumnName="countryId")
           private Country country;

  JPADataSource and HibernateDataSource support this style of declaration and will automatically handle mapping between IDs and entities.

  The example above and the following examples assume a DataSource "country" for a JPA/Hibernate entity "Country" with an Id field of "countryId", and a DataSource "city" for a JPA/Hibernate entity "City" with an Id field of "cityId".

  ### Many-To-One Relations

  An example of Many-To-One is that Many "City"s belong to One "Country". In Java, each City bean has a field of type Country. In the database, rows for cities and countries are linked by ID.

  To specify a many-to-one relation, declare a DataSourceField named after the Java field that declares the relation ("country" above) with the property `foreignKey` pointing to related DataSource's primary key:

           <field name="country" foreignKey="country.countryId"/>

  When delivered to the browser, the value of the `country` field will be the ID of the related Country entity. The `country` field can be treated as a normal text or integer field value, for example, you can use a `SelectItem` that uses `optionDataSource` to allow selecting the ID of a different related entity. Then, when the new ID is saved to the server, JPADataSource automatically looks up the related object and persists the new relation to JPA.

  NOTE:: do not declare a "type" attribute for such a field - these fields provide specialized mapping between IDs and JPA/Hibernate entities, so don't really have a single concrete type.

  If you want fields from a related entity to be included whenever your entity is fetched, for example, whenever a city is fetched you want the `countryName` fetched from the related `country` entity, use `DataSourceField.includeFrom`.

  Automatic Criteria translation

  If criteria are submitted for a ManyToOne relation field containing an ID value, this will correctly return Records that are associated with the related object that has that ID.

  For example, given a countryId of "1", you can fetch all city Records that are related to that countryId as follows:

           DataSource.get("city").fetchData(new Criteria("countryId", "1"), );

  ### One-to-Many Relations

  An example of One-To-Many relation is that One "Country" has Many "City"'s. Each "Country" has a list of cities within it, which may be declared as a Java bean property of Collection type (or List, Set, etc).

  To specify a one-to-many relation, declare a DataSourceField that:

  - is named after the Java field that declares the OneToMany relation (whose type is a Collection of the related entities)
  - declares its "type" to be the ID of the related DataSource
  - declares a `foreignKey` pointing to the related DataSource's primaryKey field
  - sets multiple="true"
  For example, for a Country bean that has a Collection of City beans:

            <field name="cities" type="city" multiple="true" foreignKey="city.cityId"/>

  With this declaration, whenever Records are loaded from the Country DataSource, they will contain a list of City Records as subobjects, accessible via `countryRecord.getAttributeAsRecordList("cities")` countryRecord.cities

  If loading all related "city" records is desirable in some circumstances and not others, you can use `OperationBinding.outputs` to avoid loading "city" records for certain operations.

  ### Many-To-Many Relations

  An example of Many-To-Many relation is that Students have multiple Courses and each Course has multiple Students. In Java each Student bean has a list of Courses and each Course bean has a list of Students. In database tables are linked using additional table holding references to both students and courses.

  To set up Many-To-Many relation between data sources you need to set up One-To-Many relation on both sides.

  For example students DataSourceField for CourseDS data source:

            <field name="students" type="integer" foreignKey="StudentDS.id" multiple="true" />

  and courses DataSourceField for StudentDS data source:

            <field name="courses" type="integer" foreignKey="CourseDS.id" multiple="true" />

  Note that type attribute can be safely omitted here.

  Note that alternative type declaration to be ID of related data source (as in regular One-To-Many relation case) would work as expected, but is not recommended to use, cause it would result in getting lots of copies of same data. Smartclient server will prevent infinite loops, but still lots of unnecessary data will be sent to client.

  ### Alternative: Many-To-One loading complete related object

  For a Many-To-One relation, instead of loading just the ID of the related object, you can load the entire related object as a nested Record. To do so, just declare the type of the field to be the ID of the related DataSource, rather than leaving type unset:

            <field name="country" type="country" foreignKey="country.countryId"/>

  The nested "country" Record will be available on a "city" record via `cityRecord.getAttributeAsRecord("country")` . Saving a City record that contains a nested Country record in the "country" attribute will result in the Country being updated in JPA/Hibernate.

  This mode is not typically used, since loading just the ID of the related Country object is more efficient if many cities are being loaded, and the related Country object can always be loaded with a second fetchData() data, which can still be done in a single HTTP request via `queuing`.

  ### Alternative: One-To-Many loading related IDs

  For a One-To-Many relation, instead of loading the complete list of related objects, you can load just a list of their IDs. To do so, just omit the type declaration when declaring the relation:

           <field name="cities" multiple="true" foreignKey="city.cityId"/>

  When saving, if a replacement list of IDs is included in the Record, the appropriate JPA/Hibernate relationships will be updated.

  This is very rarely used, and would typically only be used by client-side code that plans to programmatically work with the list of related IDs rather than display them in a UI component.

  ### NOTE: Bidirectional relations

  When relations are declared, JPA and Hibernate consider only one of the two entities to be the "owner" of the relation, meaning essentially that the references that make up the relationship are stored with that entity. When performing updates, make sure you update the entity that "owns" the relation. All changes to relations on "non-owning" entities are silently discarded.

  ### Search criteria on One-to-Many and Many-to-Many relations

  The following search operators are supported with the behaviors listed below. For simple Criteria, criteria values are treated identically to the "equals" operator and the `textMatchStyle` is ignored.

  Examples are given in terms of a "country" DataSource that has a one-to-many relation with a "city" DataSource through a relation field called "cities"

  Operator | Behavior
  `isNull` | matches country records which have no related cities
  `notNull` | matches country records which have at least one related city
  `equals, notEqual` | `criterion.value` should be a primaryKey value for the related city DataSource. This criterion matches any country which contains the passed city (or for "notEqual", matches any country which does not contain the passed city.
  `inSet, notInSet` | `criterion.value` should an array of primaryKey values for the related city DataSource. This criterion matches any country which contains any of the passed cities (or for "notInSet", which contains none of the passed cities).

  The following is an example of a criterion for matching a "country" records which have related cities with primaryKey values 1, 2 or 3 (shown serialized as JSON):

          {fieldName:"cities", operator:"inSet", value:[1,2,3]}

  As an alternative syntax, "equals", "notEqual", "inSet" and "notInSet" will also allow `criterion.value` to be specified as a list of Objects, each containing the primaryKey field and its value:

             {fieldName:"cities",operator:"inSet",value:[{cityId:1},{cityId:2},cityId:3]}

  The operators explained above work the same way for Many-To-Many relation fields.

  Any other operator applied to a relation field will cause a warning to be logged, and will be treated as though the criterion were not present (matches all records).