JPA - Persistence Operations
- JPA - Entity Managers
- JPA - Create Employee Example
- JPA - Update Employee Example
- JPA - Find Employee Example
- JPA - Delete Employee Example
- JPA - Criteria API
JPA - JPQL
- JPA - JPQL
- JPA - Scalar Function
- JPA - Aggregate Function
- JPA - Between Keyword
- JPA - Like Keyword
- JPA - Order By Clause
- JPA - Named Query
JPA - Advanced Mappings
- JPA - Advanced Mappings
- JPA - Single Table Strategy
- JPA - Joined Table Strategy
- JPA - Table per Class Strategy
JPA - Entity Relationships
- JPA - Entity Relationships
- JPA - @ManyToOne Relationships
- JPA - @OneToMany Relationships
- JPA - @OneToOne Relationships
- JPA - @ManyToMany Relationships
JPA - Useful Resources
JPA - Entity Relationships
Overview
This chapter takes you through the relationships between Entities. Generally the relations are more effective between tables in the database. Here the entity classes are treated as relational tables (concept of JPA), therefore the relationships between Entity classes are as follows −
@ManyToOne Relation
Many-To-One relation between entities − Where one entity (column or set of columns) is/are referenced with another entity (column or set of columns) which contain unique values. In relational databases these relations are applicable by using foreign key/primary key between tables.
Let us consider an example of relation between Employee and Department entities. In unidirectional manner, i.e.from Employee to Department, Many-To-One relation is applicable. That means each record of employee contains one department id, which should be a primary key in Department table. Here in the Employee table, Department id is foreign Key.
The diagram explains Many-To-One relation as follows:
@OneToMany Relation
In this relationship each row of one entity is referenced to many child records in other entity. The important thing is that child records cannot have multiple parents. In a one-to-many relationship between Table A and Table B, each row in Table A is linked to 0, 1 or many rows in Table B.
Let us consider the above example. If Employee and Department is in a reverse unidirectional manner, relation is Many-To-One relation.
@OneToOne Relation
In One-To-One relationship, one item can belong to only one other item. It means each row of one entity is referred to one and only one row of another entity.
Let us consider the above example. Employee and Department in a reverse unidirectional manner, the relation is One-To-One relation. It means each employee belongs to only one department.
@ManyToMany Relation
Many-To-Many relationship is where one or more rows from one entity are associated with more than one row in other entity.
Let us consider an example of relation between Class and Teacher entities. In bidirectional manner, both Class and Teacher have Many-To-One relation. That means each record of Class is referred by Teacher set (teacher ids), which should be primary keys in Teacher table and stored in Teacher_Class table and vice versa. Here, Teachers_Class table contains both foreign Key fields.