Persistence a child object with existing parent one

Assume there are two entities with ManyToOne relation:

@Entity
public class A {
    private long code;

    @Id
    @GeneratedValue(strategy=GenerationType.AUTO)   
    public long getCode() {
        return code;
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    public viod setCode(long code) {
        this.code = code;
    }

    // additional setters and getters
}

@Entity
public class B {
    private long code;
    private A a;

    @Id
    @GeneratedValue(strategy=GenerationType.AUTO)   
    public long getCode() {
        return code;
    }

    public viod setCode(long code) {
        this.code = code;
    }   

    @ManyToOne(fetch=FetchType.LAZY)
    @JoinColumn(name="acode", nullable=false)   
    public A getA() {
        return a;
    }

    public void setA(A a) {
        this.a = a
    }

    // additional setters and getters   
}

As part of the business flow I have code value of the existing A object and I need to insert new B object and relate it to the existing A. This code works fine:

// inject em
A a = em.find(A.class, code);
B b = new B();
b.setA(a);
em.persist(b);

My problem is with "A a = em.find(A.class, code)" line. This looks like for me as a redundant query because the B table contains a foreign key (acode). In order to improve performance I tried to create A object with an existing code value:

A a = new A();
a.setCode(code);
B b = new B();
b.setA(a);
em.persist(b);

but it doesn't work. Is there any way to avoid unnecessary query?

There is a special getReference() method that can be used instead of find() in exactly this scenario. It creates a proxy object with the specified id without issuing queires. Existence of A with that id is checked by foreign key constraint at the database side when saving B.

A a = em.getReference(A.class, code);
B b = new B();
b.setA(a);
em.persist(b);