Java Persistence/Relationships

• A relationship is a reference from one object to another.
•  In Java, relationships are defined through object references (pointers) from a source object to the target object. 
• In a relational database relationships are defined through foreign keys. The source row contains the primary key of the target row to define the relationship (and sometimes the inverse). A query must be performed to read the target objects of the relationship using the foreign key and primary key information.
• In Java, if a relationship is to a collection of other objects, a Collection or array type is used in Java to hold the contents of the relationship. In a relational database, collection relations are either defined by the target objects having a foreign key back to the source object's primary key, or by having an intermediate join table to store the relationship (both objects' primary keys).

JPA Relationship Types

1. OneToOne - A unique reference from one object to another, inverse of a OneToOne.
2. ManyToOne - A reference from one object to another, inverse of a OneToMany.
3. OneToMany - A Collection or Map of objects, inverse of a ManyToOne.
4. ManyToMany - A Collection or Map of objects, inverse of a ManyToMany.
5. Embedded - A reference to a object that shares the same table of the parent.
6. ElementCollection - JPA 2.0, a Collection or Map of Basic or Embedable objects, stored in a separate table.