MongoDB doctrine ODM OneToMany not resolving to DB ref

Question

YAML mapping doc User

User
    slug:
      unique:
        order: desc
      type: string
    roles:
            reference: true
            type: many
            targetDocument: Role
            cascade: all

Role

Role:
  type: document
  collection: Role
  fields:
    name:
        id: true
        strategy: none
        type: string 

Now adding new role to that user updating a blank object without storing any DB ref and no error. $user->getRoles() returns array of Role object. As reference are by default stored as DbRef, i was expecting the entry would be a DbRef. But it is not in this case.

 $updateApprove = $this->persistantManager->createQueryBuilder('User')
                ->field('slug')
                ->equals($user->getSlug())
                ->findAndUpdate()
                ->returnNew()
                ->field('roles');


            $updateApprove->set($user->getRoles());
            $updatedUser = $updateApprove->getQuery()
                ->execute();

Data in db after for roles executing field is showing as

 "roles" : [ 
        {}, 
        {}
    ]