How To Use Entity Sub-graphs In Spring Boot

Note: In a nutshell, entity graphs (aka, fetch plans) is a feature introduced in JPA 2.1 that help us to improve the performance of loading entities. Mainly, we specify the entity’s related associations and basic fields that should be loaded in a single SELECT statement. We can define multiple entity graphs for the same entity and chain any number of entities and even use sub-graphs to create complex fetch plans. To override the current FetchType semantics there are properties that can be set:

Fetch Graph (default), javax.persistence.fetchgraph
The attributes present in attributeNodes are treated as FetchType.EAGER. The remaining attributes are treated as FetchType.LAZY regardless of the default/explicit FetchType.

Load Graph, javax.persistence.loadgraph
The attributes present in attributeNodes are treated as FetchType.EAGER. The remaining attributes are treated according to their specified or default FetchType.

Nevertheless, the JPA specs doesn't apply in Hibernate for the basic (@Basic) attributes.. More details here.

Description: This is a sample application of using entity sub-graphs in Spring Boot. There is one example based on @NamedSubgraph and one based on the dot notation (.) in an ad-hoc entity graph.

Key points:

• define three entities, Author, Book and Publisher (Author and Book are involved in a lazy bidirectional @OneToMany relationship, Book and Publisher are also involved in a lazy bidirectional @OneToMany relationship; between Author and Publisher there is no relationship)

Using @NamedSubgraph

• in Author entity define an entity graph via @NamedEntityGraph; load the authors and the associatated books and use @NamedSubgraph to define a sub-graph for loading the publishers associated with these books
• in AuthorRepository rely on Spring @EntityGraph annotation to indicate the entity graph defined at the previous step

Using the dot notation (.)

• in PublisherRepository define an ad-hoc entity graph that fetches all publishers with associated books, and further, the authors associated with these books (e.g., @EntityGraph(attributePaths = {"books.author"}).

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