const studentObs = Rx.Observable.from([

  {"ID" : 1 , "SchoolCode": "A150", "UserID": 1 }, 

  {"ID" : 5 , "SchoolCode": "A140" , "UserID": 4}, 

  {"ID" : 9 , "SchoolCode": "C140" , "UserID": 3}

]);

const userObs = Rx.Observable.from([

  {"ID" : 1, "Name": "Rick" , "FamilyName": "Grimes" },

  {"ID" : 3, "Name": "Tom" , "FamilyName": "Cruise" },

  {"ID" : 4, "Name": "Amy" , "FamilyName": "Poehler" }

]);

function getUser(userID) {

  return userObs.filter(user => user.ID === userID);

}

​

studentObs

  .switchMap(student => {

     return getUser(student.UserID).map(user => {

       return Object.assign(student, {user: user});

     })

  })

  .subscribe(val => console.log(val));