Mongodb aggregate every two hours - mongodb

Question

I have an aggregate query of the following form
db.mycollection.aggregate([
{
"$match":
{
"Time": { $gte: ISODate("2016-01-30T00:00:00.000+0000") }
}
},
{
"$group":
{
"_id":
{
"day": { "$dayOfYear": "$Time" },
"hour": { "$hour": "$Time" }
},
"Dishes": { "$addToSet": "$Dish" }
}
},
{
"$group":
{
"_id": "$_id.hour",
"Food":
{
"$push":
{
"Day": "$_id.day",
"NumberOfDishes": { "$size":"$Dishes" }
}
}
}
},
{
"$project":
{
"Hour": "$_id",
"Food": "$Food",
"_id" : 0
}
},
{
"$sort": { "Hour": 1 }
}
]);
Instead of doing this as above in one hour durations e.g. 0-1,1-2,2-3,3-4,4-5,...,23-24, I want to be able to do this in two hour durations. e.g. 0-2,2-4,4-6,...,22-24. Is there a way to do that?

Answer 1

Hint: use arithmetic aggregation operators in $project
Lets say, H=floor(hour/2), where hour is actual hour from document date. Then you can get H by applying $floor and $divide operators to this date
"H": { $floor: { $divide: [ { "$hour": "$Time" }, 2 ] } }
Here H corresponds to the pair of hours (Hours=[0,2) => H=0, Hours=[2,4) => H=1, Hours=[22,24) => H=11, etc.) and you can pass it to the $group stage with
$group: { "_id": { "day": { $dayOfYear: "$Time" }, "H": "$H" } }
Then you can output the pair of hours for specific H with
"Hours": [ { $multiply: [ "$H", 2 ] }, { $sum: [ { $multiply: [ "$H", 2 ] }, 2 ] } ]
Given collection of documents
{ "Time" : ISODate("2016-01-30T01:00:00Z"), "Dish" : "dish1" }
{ "Time" : ISODate("2016-01-30T02:00:00Z"), "Dish" : "dish2" }
{ "Time" : ISODate("2016-01-30T03:00:00Z"), "Dish" : "dish3" }
{ "Time" : ISODate("2016-01-30T04:00:00Z"), "Dish" : "dish4" }
{ "Time" : ISODate("2016-01-30T05:00:00Z"), "Dish" : "dish5" }
{ "Time" : ISODate("2016-01-30T06:00:00Z"), "Dish" : "dish6" }
{ "Time" : ISODate("2016-01-30T07:00:00Z"), "Dish" : "dish7" }
{ "Time" : ISODate("2016-01-30T08:00:00Z"), "Dish" : "dish8" }
{ "Time" : ISODate("2016-01-30T09:00:00Z"), "Dish" : "dish9" }
and using the next aggregate on it
db.mycollection.aggregate([
{
"$match":
{
"Time": { $gte: ISODate("2016-01-30T00:00:00.000+0000") }
}
},
{
"$project":
{
"Dish": 1,
"Time": 1,
"H": { $floor: { $divide: [ { $hour: "$Time" }, 2 ] } }
}
},
{
"$group":
{
"_id":
{
"day": { $dayOfYear: "$Time" },
"H": "$H"
},
"Dishes": { $addToSet: "$Dish" }
}
},
{
"$group":
{
"_id": "$_id.H",
"Food":
{
"$push":
{
"Day": "$_id.day",
"NumberOfDishes": { $size: "$Dishes" }
}
}
}
},
{
"$sort": { "_id": 1 }
},
{
"$project":
{
"Hours": [ { $multiply: [ "$_id", 2 ] }, { $sum: [ { $multiply: [ "$_id", 2 ] }, 2 ] } ],
"Food": "$Food",
"_id": 0
}
}
]);
provides the result
{ "Food" : [ { "Day" : 30, "NumberOfDishes" : 1 } ], "Hours" : [ 0, 2 ] }
{ "Food" : [ { "Day" : 30, "NumberOfDishes" : 2 } ], "Hours" : [ 2, 4 ] }
{ "Food" : [ { "Day" : 30, "NumberOfDishes" : 2 } ], "Hours" : [ 4, 6 ] }
{ "Food" : [ { "Day" : 30, "NumberOfDishes" : 2 } ], "Hours" : [ 6, 8 ] }
{ "Food" : [ { "Day" : 30, "NumberOfDishes" : 2 } ], "Hours" : [ 8, 10 ] }