@Entity
public class MyObject {

    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    private long id;

    @ManyToOne(fetch = FetchType.LAZY)
    private final MySource source;

    public MyObject(MySource source) {
        this.source = source;
    }

    protected MyObject() {
        this.source = null;
    }

    public long id() {
        return this.id;
    }

    public MySource source() {
        return this.source;
    }

    @Override
    public boolean equals(Object obj) {
        boolean equals = this == obj;

        if (!equals && obj instanceof MyObject) {
            MyObject other = (MyObject) obj;
            equals = other.id() == id;
        }

        return equals;
    }

    @Override
    public int hashCode() {
        return (int) id % 16;
    }

}


@Entity
public class MySource {

    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    private long id;

    public MySource(String name) {
        this.name = name;
    }

    protected MySource() {
        this.name = "";
    }

    @Column
    private final String name;

    public long id() {
        return id;
    }

    public String name() {
        return this.name;
    }

    @Override
    public boolean equals(Object obj) {
        boolean equals = this == obj;

        if (!equals && obj instanceof MySource) {
            MySource other = (MySource) obj;
            equals = other.id() == id;
        }

        return equals;
    }

    @Override
    public int hashCode() {
        return (int) id % 16;
    }

}

public void removeById(long id) {
    em.getTransaction().begin();
    MyObject myObjectRef = em.getReference(MyObject.class, id);
    em.remove(myObjectRef);
    em.getTransaction().commit();
    em.clear();
}