Difference between array of objects in Angularjs

I'm using Angular. I'm trying to compare two arrays of objects I was able to get it working doing it like this:

  var compareUsers = function () {
    //Comparing assigned groups with all to return available users
    var assignedUsersIds = {};
    var usersIds = {};
    availableUsers = []; //declared higher up, populated with user objects on load

    //assignedUsers, declaired higher up, populated by function calling compareUsers
    assignedUsers.forEach(function (el, i) {
      assignedUsersIds[el.id] = assignedUsers[i];
    });

    allUsers.forEach(function (el, i) {
      usersIds[el.id] = allUsers[i];
    });

    for (var i in usersIds) {
        if (!assignedUsersIds.hasOwnProperty(i)) {
            availableUsers.push(usersIds[i]);
        }
    };
    console.log(availableUsers);
    return availableUsers;
  }

I found a better way to do it so I refactored it to this, using lodash:

  var compareUsers = function () {
    availableUsers = _.filter(allUsers, function(user){
      return !_.findWhere(assignedUsers, user);
    });
    console.info(availableUsers);
    return availableUsers;
  }

However, I'm not getting the correct results and not sure what I messed up. The new methods returns availableUsers which are in the assignedUsers list for some groups. The first time it runs, it seems to work but if I keep changing what group i'm looking at the results are all off and don't add up.

I found this method here.