title: Loopback - Add existent object to has many property via id
body: <p>I want to relate two instance objects <code>A</code> and <code>B</code>. In my A model definition, <code>A hasMany B</code>. </p>
  
  <p>Now loopback default endpoints let me create a new B object related to A, but what happens when I do not want to create a new object, I just want to relate two existent objects.</p>
  
  <p>I would like to post the relation, not the object. Is this posible?</p>