How to Get the Max Daily Value per Week by Grouping - mongodb

Question

I am making a project which requires me to first calculate how much distance was traveled per day. and then on that data I have to how show What was the maximum, minimum and average distance traveled that particular week?
This is a mongoDB script I have written.
db = connect("localhost:27017/mydb");
var result = db.trips.aggregate([
{
"$unwind" : "$trips"
},
{
"$match" : {
"trips.startTime" : {"$lte" : ISODate("2015-10-31T23:59:59Z"), "$gte" : ISODate("2015-10-25T00:00:00Z")}
}
},
{
"$group" :
{
"_id" : {
"date" : {"$dayOfMonth" : "$trips.startTime"}
},
"distance" :{"$sum" : "$trips.distance"}
}
}
]);
while(result.hasNext())
{
print(tojson(result.next()));
}
Which when replaced by dynamic dates gives me correct values.
Now it leaves me with two options, either I modify the current group query or write a double group query. Double group query seems a more valid approach. My attempt at writing such a query.
{
"$group" :
{
"_id" : {
"week" : "$_id.date"
},
"max-distance" : {
"$max" : "$distance"
}
}
}
Adding these lines didn't make a difference, clearly I know I am doing wrong, but how to correct it. i would need help with that
Thanks

Answer 1

You seem to wan the $week operator, but of course you need a valid Date as input in order to extract the "week" from that.
What you may not know is that you can instead use "date math" to round out the date to a "day", where the result is still a Date object. Then you can use the $week operator to obtain your $max values:
db.trips.aggregate([
{ "$unwind" : "$trips" },
{ "$match": {
"trips.startTime" : {
"$lte": ISODate("2015-10-31T23:59:59Z"),
"$gte": ISODate("2015-10-25T00:00:00Z")
}
}},
{ "$group": {
"_id": {
"$add": [
{ "$subtract": [
{ "$subtract": [ "$trips.startTime", new Date(0) ] },
{ "$mod": [
{ "$subtract": [ "$trips.startTime", new Date(0) ] },
1000 * 60 * 60 * 24
]}
]},
new Date(0)
]
},
"distance": { "$sum": "$trips.distance" }
}},
{ "$group": {
"_id": { "$week": "$_id" },
"max-distance": { "$max": "$distance" }
}
]);
The basic trick in the first part is when you $subtract one Date object from another, the result is the millseconds in difference. So using the epoch date the data is converted to it's milliseconds equivalent and then you can use the math to round that number to a day.
(1000 * 60 * 60 * 24) is the number of milliseconds in a day, so finding the modulo ( $mod ) of that returns the remainder of milleseconds past the day, which you can subtract from the date value in the document to round to a day.
The same is true of $add when adding a Date object to a number, the result is a Date. So this handles the conversion, and then the $week can be extracted from there.