how to persist foreign key only if not exist?

[rrlangly]

The Car table is a list of cars, all unique.
The Engine table is a list of engines, all unique.
The Report table has a reference to the Car table as shown below. There can be many reports, but I don't want to duplicate anything in the Car or Engine tables.

My problem is I want to insert a Report, but only insert a car into the Car table if its not already in the Car table. Currently, all this works with the exception that Cars are being duplicated into the Car table when I add a report.

Any help much appreciated.

	ireport = (IReportBean) (new InitialContext()).lookup("java:comp/env/ejb/ReportBean");

	Car car = new Car(c);
	Report report = new Report(r);
	report.addCar(car);

	ireport.addReport(report);

And my EntityManager basically does ...

	em.persist(report);
	em.flush();

My table structure ...

CREATE SEQUENCE report_seq START 1000;
CREATE TABLE Report (
    id          INTEGER DEFAULT NEXTVAL('report_seq') PRIMARY KEY,
    car_id      INTEGER REFERENCES Car (id),
    condition   VARCHAR(32) NOT NULL
);

CREATE SEQUENCE car_seq START 1000;
CREATE TABLE Car (
    id          INTEGER DEFAULT NEXTVAL('car_seq') PRIMARY KEY,
    engine_id   INTEGER REFERENCES Engine (id),
    mfg         VARCHAR(32) NOT NULL
);

CREATE SEQUENCE engine_seq START 1000;
CREATE TABLE Engine (
    id          INTEGER DEFAULT NEXTVAL('engine_seq') PRIMARY KEY,
    year        VARCHAR(32) NOT NULL,   
    description VARCHAR(128) DEFAULT NULL
);

Annotations used in my Report entity.

@SequenceGenerator(name="reportIdGen", initialValue=1000, allocationSize=50,
    sequenceName="report_seq")
public class Report implements Serializable {
    @Id
    @Column(name="id")
    @GeneratedValue(strategy=GenerationType.SEQUENCE, generator="reportIdGen")
    private Integer id;

    @ManyToOne(cascade=CascadeType.PERSIST)
    @JoinColumn(name="car_id")
    private Car carId;

    @Column(name="condition")
    private String condition;

	// ...