mongoDB - Update query adding an array document to an existing document - node.js

Question

The userlist collection contains documents of the following format.
{
"_id" : ObjectId("5381d32ce72cc794166eede2"),
"name" : "Haseeb",
"password" : "dgkhan",
"email" : "hasseeb#yahoo.com",
"address" : "237 D, Faisal Town , Lahore",
"phone" : "5162806"
}
I intend to add another member in the existing document such that the resultant document looks like this.
{
"_id" : ObjectId("5381d32ce72cc794166eede2"),
"name" : "Haseeb",
"password" : "dgkhan",
"email" : "hasseeb#yahoo.com",
"address" : "237 D, Faisal Town , Lahore",
"phone" : "5162806",
"purchases" : [{
"itemID": xyz,
"quantity": 142
},
{
"itemID": kjh,
"quantity": 987
}
}]
}
For this I have written the following mongoskin query, but it is not performing any updates.
db.collection('userlist').update(
{_id:req.session._id},
{
'$push': { purchases: {
itemID: item.ID,
quantity: item.quantity
}
}
}, function(err) {
if (err) throw err;
console.log('Updated!');
});
}
Value of req.session._id = 5381d32ce72cc794166eede2 i.e a valid _id field of a docuemnt in my collection whereas item.ID and item.quantity are also valid strings.
Any help would be really appreciated.

Answer 1

I'm more familiar with Mongoose, but looking at the docs you could probably either use:
collection.updateById(req.session._id, ...)
or
collection.update({_id: toObjectID(req.session_id)}, ...)

Answer 2

I think you should be using $set instead of $push.
A good resource for this specific question is: http://docs.mongodb.org/manual/reference/method/db.collection.update/#db.collection.update

Answer 3

Something like this. You need to include the $each syntax.
db.collection('userlist').update(
{_id:req.session._id},
{
$push { purchases: { $each: {
itemID: item.ID,
quantity: item.quantity
}
}
}
}, function(err) {
if (err) throw err;
console.log('Updated!');
});
}