Part 2, Chapter 11

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Table of content

This chapter explains JPA relationships from a data access performance point of view.

11. Relationships
11.1 Relationship types     
11.2 @ManyToOne       
11.3 @OneToMany       
11.3.1 Bidirectional @OneToMany   
11.3.2 Unidirectional @OneToMany  
11.3.3 Ordered unidirectional @OneToMany
11.3.3.1 @ElementCollection    
11.4 @OneToOne      
11.4.1 Unidirectional @OneToOne   
11.4.2 Bidirectional @OneToOne  
11.5 @ManyToMany      
11.5.1 Unidirectional @ManyToMany 
11.5.2 Bidirectional @ManyToMany  
11.5.3 The @OneToMany alternative

Chapter summary

Entity relationships are very common in enterprise applications using JPA as a data persistence technology.
From a database perspective, table relationships can be:

• one-to-many
• one-to-one
• many-to-many

But JPA defines multiple annotations for mapping these table relationships:

• @ManyToOne
• @OneToMany
• @OneToOne
• @ManyToMany
• @ElementCollection

Unlike the table relationships which have the foreign key on one side only, JPA relationships can be either unidirectional or bidirectional. Unfortunately, not all JPA associations are efficient and this chapter aims to unravel which relationships are useful and which ones you shouldn’t use in a high-performance data access system.

As a rule of thumb, always choose a JPA association that’s based on a child-side @ManyToOne or @OneToOne mapping because that’s the most natural way of representing the foreign key side of a table relationship.

You should always remember that JPA relationships are not a mandatory requirement and sometimes a JPQL query is a much more efficient alternative to a collection mapping.

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