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Listing 4.13 Relations between entities should be bidirectional
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@Entity @Indexed public class Item { @ManyToMany
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Mapping relationships between entities
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@IndexedEmbedded private Set<Actor> actors; @ManyToOne @IndexedEmbedded private Director director; ... } Entity @Indexed public class Actor { @Field private String name; @ManyToMany(mappedBy="actors") @ContainedIn private Set<Item> items; ... } @Entity @Indexed public class Director { @Id @GeneratedValue @DocumentId private Integer id; @Field private String name; @OneToMany(mappedBy="director") @ContainedIn private Set<Item> items; ... }
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Embed actors when indexing Embed director when indexing
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contained B actor is index in item
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director is contained in item index
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@ContainedIn B is paired with an @IndexedEmbedded annotation on the other side of the relationship. @ContainedIn can be placed on both collections and single-value
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associations whether or not the association is the owning side of the bidirectional relationship. When Hibernate Search finds a change in an entity having an @ContainedIn association (director in our example), it marks the associated entity instance(s) as changed (items contained in director.items in our example). One or more Lucene document updates will then be triggered. Some people are confused by @IndexedEmbedded and @ContainedIn. They don t know which side needs to be marked as @IndexedEmbedded to enable the query they want. Think about it this way: @IndexedEmbedded is the property you can navigate to in your queries; @ContainedBy is not.
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Sometimes, it s inconvenient to have to make a relationship bidirectional to please Hibernate Search. If the associated entity never changes in your system (immutable entity), you don t have to add @ContainedIn. Since no changes will happen behind Hibernate Search s back, your index will be kept synchronized. If the associated entity changes, but you cannot afford a bidirectional relationship, it s always possible to trigger a manual reindexing of the owning entity (see section 5.4). Taking listing 4.13 as an example, you could decide to manually reindex all the item entities every night. Or you could keep track of the actor changes and cherry-pick the item entities that need to be reindexed using an HQL query.
Mapping more advanced data structures
It s not uncommon to have nested associations: embedded entities containing embedded relationships. We can even imagine these embedded relationships pointing to other entities that have embedded relationships and so on. You should avoid embedding too much information in a single Lucene document because indexing takes longer and the index directory grows bigger. A Lucene document should contain only the necessary bits of information to express planned queries. This poses the question of how to stop embedding associations and at which level to stop. By default, Hibernate Search stops embedding associations in a given object s graph branch when the same class has already been processed. Figure 4.5 describes the default strategy. Hibernate Search raises an exception in this situation to prevent infinite loops created by circular relationships. The default behavior won t always match your needs:
It s common to have a class associated with itself that needs to be indexed (parent-child relationships from the same class are a good example). Entities that are both embedded and root indexed could lead to deep indexed object graphs. For example, the Actor entity is embedded in the Item entity but is also indexed separately because we d like to be able to search for actors specifically.
@IndexedEmbedded allows you to control the depth at which association embedding stops. By default, the depth is not limited, and the method described in figure 4.5 applies. A depth limit is defined per association; it s the maximum amount of embedding allowed in the branch started by the association (including the current association). Figure 4.6 is a revised version using an explicit depth limit. In figure 4.6, each association is either marked with an explicit depth or left at the default depth (infinite). The upper branch shows that from entity A, Hibernate Search is allowed to embed only two associations in depth for that branch. B is then included. The association between B and C indicates that the depth from this association cannot be higher than three. The association from C to B is not embedded because the maximum number of jumps allowed by the association from A to B was
Figure 4.5 By default Hibernate Search raises an exception when embedding an association if a class has already been processed in a given branch.
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