Is it possible to not persist associated entities?

I have a table

create table public.B(    
  id character varying(30) not null,    
  name character varying(255) not null,   
  a_id character varying(6) not null,
  primary key (id, a_id),
  foreign key (a_id) references public.A (id)
  match simple on update no action on delete no action
);

With This entity

@Entity(name = "B")
@IdClass(BId.class)
public class B {
    @Id
    @NotBlank
    public String id;

    @NotBlank
    public String name;

    @Id
    @ManyToOne(optional = false)
    @JoinColumn(name = "a_id", nullable = false)
}

And associated Entity

@Entity(name = "A")
public class A {
    @Id
    @NotBlank
    public String id;

    @NotBlank
    public String name;
}

I just want persist the entity B and the foreign key but not persist the entity A

Example

void myfunction(final String aId){
  if (aId != null) {
    var a = new A();
    a.id = aId;
    var b = new B();
    b.id = "pop";
    b.name = "pop";
    b.a = a;
    entityManager.persist(b);
  }
}

I want to persist b without create A in database (because it already exists In database). Id just want persist in column a_id the id.

It ‘s work if before I retrieve the entity A over entityManager but it’s doing a request in database useless in my case. I just want to instert.