How to represent an association "many to many" with an association class?

Question

I work with JPA (and TopLink essentials).
Class Person
Class Project
Association M:N between these 2 classes. A person has a function in each project she participates in.
My relational schema:
table PERSON, primary key person_id, person_name,...
table PROJECT, primary key project_id, project_name,...
table PARTICIPATION,
- primary key (person_id, project_id). person_id references PERSON and project_id references PROJECT.
- column "function".
How to write the entities Person, Project and Participation so that they map the relational schema? I don't know what to put as the "id" (@Id) of the class Participation
I know how to solve the problem by adding an attribute id (annotated by @Id) in the class Participation that would map to an id column in the table PARTICIPATION but I would like to match exactly the relational schema: the primary key of PARTICIPATION is (person_id, project_id) and not another id column.
Thanks in advance for your answers.

Answer 1

Thanks for your answer. I see what I have to do.
There are 2 items I don't understand in your message. My questions are beneath, inside the text of your message:
You have a m-m relationship with additional
information in the join table. To map this you need
an object representing the join table, in your case
you have chosen Participation. So you basically need
to define a 1-m from Person to Participation and a
1-m from Project to Participation (unless you only
care about access from one side).
Participation should have a 1-1 to Person and another
1-1 to Project using the two foreign key fields.I would say they are m-1 associations. But perhaps you have made a typing error?
Although TopLink allows 1-1 mappings to be used a
primary keys, the JPA Spec requires that you have
basic mappings, so you also need a personId and
projectId attribute in the Participation.I don't understand. Can you detail your answer?
I have not found the section of the JPA specification which says how to do with 2 mappings.
>
Because you will have two mappings for the same
foreign key field, in TopLink you will require to set
one of them read-only, you do this in JPA through
setting insertable and updateable on the @Column to
false.