@Entity("user")
public class User{
     @Id
     private ObjectId id;
     private String username;

     public void setId(ObjectId id){
           this.id = id;
     }
     public ObjectId getId(){
           return this.id;
     }
     public void setUsername(String username){
           this.username = username;
     }
     public String getUsername(){
          return this.username;
      }
}

@Entity("assistant")
public class Assistant extends User{
       private String fullname;
       public String getFullname(){
              return this.fullname;
       }
       public void setFullname(String fullname){
              this.fullname = fullname;
       }
}