Users = [{
    _id: 1,
    name: "Sam",
    email: "sam@gmail.com",
    group: "Rangers"
  },

  {
    _id: 2,
    name: "Michael",
    email: "michael@gmail.com"
    group: "Muse"
  },

  {
    _id: 3,
    name: "John",
    email: "john@gmail.com"
    group: "Merchant"
  },
  .....
]

Items = [
  {
    _id: 1,
    user_id: 1,
    item_name: "Flying Sword",
    timestamp: ...
  },
  {
    _id: 3,
    user_id: 1,
    item_name: "Invisible Cloak",
    timestamp: ...
  },

  {
    _id: 4,
    user_id: 2,
    item_name: "Iron Shield"
  },

  {
    _id: 5,
    user_id: 7,
    item_name: "Splashing Gun",
    timestamp: ...
  },
  ...
]

Users.find({group: "Muse"}, function(err, users){
    I DON"T KNOW WHAT TO WRITE INSIDE
})

[
  {
    _id: 4,
   name: "Jack",
   email: "jack@gmail.com",
   group: "Muse",
   items: [
     {
       _id: 8
       name: "Magic Wand",
       user_id: 4,
       timestamp: ...
     }
     {
       _id: 12
       name: "Blue Potion",
       user_id: 4,
       timestamp: ...
     },

     {
       _id: 18
       name: "Teleportation Scroll",
       user_id: 4,
       timestamp: ...
     }
   ]
  }
  .....
  More USERS of similar structure



]

Users.find({group: "Muse"}, function(err, users){
  var userIDs;

  if (err) {
    // do error handling
    return;
  }

  userIDs = users.map(function (user) { return user._id; });

  Items.find({user_id: {$in: userIDs}}, function (err, items) {
    if (err) {
      // do error handling
      return;
    }

    users.forEach(function (user) {
      user.items = items.filter(function (item) {
        return item.user_id === user._id;
      });
    });

    // do something with modified users object
  });
});

Users.find({group: "Muse"}).exec().then(function(users) {
  var userIDs = users.map(function(user) {
    return user._id;
  });

  // returns a promise
  return Promise.all([
    // include users for the next `then`
    // avoids having to store it outside the scope of the handlers
    users,
    Items.find({
      user_id: {
        $in: userIDs
      }
    }).exec()
  ]);
}).then(function(results) {
  var users = results[0];
  var items = results[1];

  users.forEach(function(user) {
    user.items = items.filter(function(item) {
      return item.user_id === user._id;
    });
  });

  return users;
}).catch(function (err) {
  // do something with errors from either find
});

Users.find({group: "Muse"}).exec().then(function(users) {
  return bluebird.map(users, function(user) {
    return Items.find({user_id: user._id}).exec().then(function (items) {
      user.items = items;
      return user;
    });
  }, {concurrency: 5});
}).then(function(users) {
  // do something with users
}).catch(function(err) {
  // do something with errors from either find
});