Mongodb how to find how much days ago from a timestamp field - mongodb

Question

I was trying to find the number of days ago using the timestamp but i dont know how to do that ?
{
"_id" : ObjectId("5504cc9ddd5af617caae30b3"),
"session_id" : 1,
"Timestamp" : "2014-04-07T10:51:09.277Z",
"Item_ID" : 214536502,
"Category" : 0
}
How I can calculate the number of days ago using the field "Timestamp" ?

Answer 1

You may use aggregate, $project with new Date() on the Timestamp field, then do the calculation, something like this:
pipe = {
"$project" : {
"_id" : 1,
"daySince" : {
"$divide" : [
{
"$subtract" : [
new Date(),
new Date("$Timestamp")
]
},
86400000
]
}
}
}
To calculate:
db.collection.aggregate(pipeline=pipe)
Since Timestamp isn't a ISODate object, you just need to convert it to one, then subtract to current date, and divide the result by 60*60*24*1000, then it will be the number of days since today.
You can also change the new Date() to what you need to be compared.
Updated:
Since I believe the Timestamp format might be malformed, alternatively you may use mapReduce functions to calculate this:
// in your mongo shell using the db
var mapTimestamp = function() {
daySince = parseInt(new Date() - new Date(this.Timestamp) / 86400000);
emit(this._id, daySince);
}
// since you map reduce only on one field, there's really no need for this
var reduceTimestamp = function (key, value) { return value; }
db.collection.mapReduce(mapTimestamp, reduceTimestamp, {out: "sample"})
To show the results:
db.sample.find()