How can I manage to create a many-to-many relationship. The generated entity will have extra attributes.

I开发者_StackOverflow have 2 entities: Class(of students) and Student. A student can be in many classes(like in college) and a class has many students. The problem is how to ensure that this entity, generated in the middle, has 2 primary keys, the ids of each other entity (Student and Class). I need to know how to create it using annotations. I use EJB3 and JPA Annotations in the project.

First, you don't need a middle entity. You have two entities and a join table between them.

You need a middle entity only if you have additional information about the relation - for example a StudentClass may have timesAbsent column.

In case you really need the third entity, you can have:

1. an @EmbeddedId, where you define a separate class holding the two parts of the primary key. That class must be @Embeddable
2. an @IdClass which will let you specify two @Id fields. You'll again need another class to hold the two fields representing the key.

See this question for which option to choose.

Note that you thus have a composite primary key, not two primary keys (which you can't have)

I know how to make this happen using hibernate. May be it'll help.

Make the collection type Set.

 public class CollegeClass { 
     private Set<Student> students;
 }

 public class Student { 
     private Set<CollegeClass> classes; 
 }