Spring Data JPA Sample

This project was setup to be a playground to experiment with:

1. spring-data-jpa
2. Hibernate

Project Scenario

1. There are 3 tables user, user_department and user_address (Not the ideal way to organize data)
2. Use the database.sql to create the tables and insert the related data
3. There is a one to one mapping between user and user_department. The user_id column on user_department references the id column on the user table.
4. There is a one to one mapping between user and user_address. The user_id column on user_address references the id column on the user table.

What was accomplished

1. How to set establish @OneToOne relationship mapping between objects with lazy loading
2. How to use multiple NamedEntityGraphs

Concepts to be understood

1. Owning side of a relationship in hibernate This stackoverflow question: https://stackoverflow.com/questions/2749689/what-is-the-owning-side-in-an-orm-mapping helps understand these concepts This link in the comment: https://www.javacodegeeks.com/2013/04/jpa-determining-the-owning-side-of-a-relationship.html gives a very good explanation

2. Lazy loading In the example (not an ideal example) in this project. We have a User class (represents user table ) that has a 1-1 mapping with UserDepartment class (represent user_department) When querying for User alone, UserDepartment entities should not be fetched unless specified. To achieve this fetch = FetchType.LAZY and optional=false need to be specified in the @OneToOne annotation that is not on the owning side of the relationship By default spring-boot's ObjectMapper does not support lazy loading and looks for the related UserDepartment object and results in an exception being thrown The exception goes like: could not initialize proxy - no Session (through reference chain: To circumvent this we need to use an ObjectMapper that supports lazy loading That is where we need to use: https://github.com/FasterXML/jackson-datatype-hibernate This helps circumventing this issue.

   Include the dependency as:

    <dependency>
        <groupId>com.fasterxml.jackson.datatype</groupId>
        <artifactId>jackson-datatype-hibernate5</artifactId>
        <version>2.9.0</version>
    </dependency>

   Inject the Hibernate5Module as:

    @Bean
    public Module datatypeHibernateModule() {
        return new Hibernate5Module();
    }

   Although the Hibernate5Module does not look for the related entities it returns them as null.

   To get around this User class need to be annotated with @JsonInclude(JsonInclude.Include.NON_EMPTY)