Mongoose Returned Model can't be updated - node.js

Question

I'm pretty new to Mongoose/Mongo and node.js, so I suspect this is just a misunderstanding on my side, but...
The code sample below is the smallest failing example, not specifically my use case.
var User = app.db.model('User');
User.find({email: 'm8#test.com'}, function (err, models) {
models[0].update(function(err, mod) {
console.log(err.message)
});
});
This results in the following error: After applying the update to the document {_id: ObjectId('54647402cb955748153ea782') , ...}, the (immutable) field '_id' was found to have been altered to _id: ObjectId('546d9e0e539ed9ec102348f9')
Why is this happening? I would have thought calling update on the model returned from the initial find would have been fine.
Please note: in my use case there are things happening in between the find and the update. Specifically, I'm doing something similar to:
model.property.push(objectId)
Which I then want to commit via the update.
I'm sure this is a straight-forward issue, but I can't see anywhere in the docs I may be getting it wrong.
All help appreciated.
UPDATE:
What I actually needed to do was:
var User = app.db.model('User');
User.find({email: 'm8#test.com'}, function (err, models) {
models[0].save(function(err, mod) {
console.log(err.message)
});
});
Using 'save' rather than 'update'

Answer 1

I don't know if I understood
Find and Update ( for example using express )
var email = req.params.email;
User.find({email:email}, req.body, function(err,user){
if(err){
throw err;
}
//you do stuff like this
var obj = {
password:'new pass',
username:'username'
}
//use save if you want validate
User.update(user[0],obj,function(err, mod) {
console.log(err)
});
});
Only Update: ( for example using express )
User.update({email:email}, req.body, {}, function(err,user){
if(err){
throw err;
}
res.send(200, {
message : 'User updated ' + user
});
});
Remember that:
A model is a compiled version of the schema.
I hope this can help you