Wrong array order after populating

I have the problem but I can't paste whole code here so let me try to explain. I need to find some document by id and populate its ObjectId's element of array. Let's say I have following document from User collection:

{
  _id: 1,
  "first_name":"Nick",
  "last_name":"Parsons",
  "email":"nick@movementstrategy.com",
  "password":"foo",
  "clients":[
    "50f5e901545cf990c500000f",
    "50f5e90b545cf990c5000010",
    "50f5e90b545cf990c5000013"
  ]
}

After finding it I'm populate its clients like this:

User.findById(1)
  .populate('clients')
  .exec(function (err, user) {

  });

After the code above is executed I have 'user' variable with populated array of clients. But the clients have the wrong order, something like this:

       {
          _id: "50f5e901545cf990c500000f",
          "first_name":"cl1",
       },
       {
          _id: "50f5e90b545cf990c5000013",
          "first_name":"cl3",
       },
       {
          _id: "50f5e90b545cf990c5000010",
          "first_name":"cl2",
       }

Should be:

    "50f5e901545cf990c500000f",
    "50f5e90b545cf990c5000010",
    "50f5e90b545cf990c5000013" 

Instead of:

    "50f5e901545cf990c500000f",
    "50f5e90b545cf990c5000013",
    "50f5e90b545cf990c5000010"     

Does anybody has the similar problem or am I lucky?

EDIT: I've added the code: http://pastie.org/5901177 In that code I create 3 clients and pushed into user.clients array. After that I added one more client on a top of user.clients array. So when I do

  Client.where('_id').in(user.clients)

I'm getting wrong order. Look at this please via console.