If you are lucky enough to have users, some of them will eventually want help. One of the easiest ways to understand and diagnose a problem, is to impersonate (log in as) the user who needs your assistance.

Although the docs don't say how to accomplish this, there is a simple and secure way to get it working with the existing meteor API. The goal is to convince the client and the server that you are a different user by calling setUserId on both. This approach has been approved by a meteor core developer and we use it in production.

First we'll create a method called impersonate, which calls setUserId on the server if the caller is an admin.

Meteor.methods({
  impersonate: function(userId) {
    check(userId, String);

    if (!Meteor.users.findOne(userId))
      throw new Meteor.Error(404, 'User not found');
    if (!Meteor.user().isAdmin)
      throw new Meteor.Error(403, 'Permission denied');

    this.setUserId(userId);
  }
});

Next we'll write some template code that calls the method from the client. In this example, this.id is the id of the user we wish to impersonate.

Template.impersonate.events({
  'click .impersonate': function() {
    var userId = this.id;

    Meteor.call('impersonate', userId, function(err) {
      if (!err) {
        Meteor.connection.setUserId(userId);
        Router.go('home');
      }
    });
  }
});

In order to get your subscriptions to reset after swapping users, I'd recommend placing them inside of an autorun.

Tracker.autorun(function() {
  if (Meteor.userId()) {
    Meteor.subscribe('rooms');
  }
});

And that's it! Simply hit refresh or manually change the URL to stop impersonating.