Updating a nested document mongo. Concurrency issues - node.js

Question

I am struggling with a seemingly basic issue in Mongoose.
I have a method that looks like so:
changeStateOnIncident: function(req, res, next) {
Incident.findOneQ({ _id: req.params.id })
.then(function(incident) {
if (!incident) { return next(new restify.ResourceNotFoundError("Incident with id " + req.params.id + " not found.")) }
return incident.addStateChange(req.body)
.then(function(savedState) {
return res.json(201, savedState);
})
.then(function() {
var alertOrders = _.map(incident.alertees, function(alertee) {
return alertee.createAndSendAlert(req.body)
})
return Q.allSettled(alertOrders)
.then(function(results) {
return incident.save();
})
})
})
.fail(function(err) {
return next(new restify.InvalidContentError(JSON.stringify(err)))
})
},
As for createAndSendAlert and addStateChange, their definitions are here
What I am trying to do here is as follows:
Find the Incident matching the ID passed in.
Add the state event to the array of state changes.
Return that newly added change event back to the user.
For every alertee on the Incident, loop through and add an alert to that alertees list of alerts, the result of an SMS request.
Save that incident.
Unfortunately, the way I'm doing it now is failing for concurrent writes. Suppose I send two
{
_id: "53cc87d0feeb4302007eadbe",
alertees: [
{
_id: "eJ65nfJh_g",
alerts: [
{
"_id": "53cc87d1feeb4302007eadc1",
"created_at": "2014-07-21T03:24:01.304Z",
"response": "Sent",
"state": "First state change",
"updated_at": "2014-07-21T03:24:06.802Z"
},
{
"_id": "53cc87d2feeb4302007eadc5",
"created_at": "2014-07-21T03:24:02.348Z",
"response": "Sent",
"state": "Emergency",
"updated_at": "2014-07-21T03:24:06.802Z"
},
{
"_id": "53cc87d6feeb4302007eadcc",
"response": "Sent"
"state": "Ended",
}
]
}
],
states: [
{
_id: "53cc87d1feeb4302007eadbf",
state: "First state change"
},
{
_id: "53cc87d1feeb4302007eadc2",
state: "Second state change"
},
{
_id: "53cc87d2feeb4302007eadc4",
state: "Third state change"
},
{
_id: "53cc87d6feeb4302007eadcb",
state: "Fourth state change"
}
]
}
I think what is happening is IF I send two states A and B. And A's SMS response comes back before B's it will possibly invalidate the document B is trying to modify. I am losing changes to the sub document if state changes are sent too close to each other.
Any idea? How could I rewrite this? Using update methods perhaps?