Group by inner array element after $unwind - mongodb

Question

I have the following time series data stored in mongodb
{
"_id" : ObjectId("59a46062e1aeb958a712490e"),
"channelName" : "ABC",
"rtData" : [
{
"ts" : ISODate("2017-08-28T18:26:42.837Z"),
"data" : [ 676.297664, 676.297664 ]
},
{
"ts" : ISODate("2017-08-28T18:27:42.837Z"),
"data" : [ 724.297664, 676.297664 ]
},
{
"ts" : ISODate("2017-08-28T18:29:42.837Z"),
"data" : [ 878.297, 676.297 ]
}
]
}
I want to group the data based on the ts field on hour and get the first element of rtData for that hour.
Here is what I have tried
db.channels.aggregate( [ {$match: {"channelName": "ABC"} }, { $unwind : "$rtData" }, { $group : {_id: { $hour: "$rtData.ts" }, ucast: { $sum: $rtData.data[0]} }
But the above code gives me the following output
{ "_id" : 28, "ucast" : 0 }
What I actually want is
{ "_id" : 28, "ucast" : 676.297664 }

Answer 1

You don't notate getting a first element of an array in an aggregation pipeline like that. You want $arrayElemAt which returns the array value by index:
db.channels.aggregate( [
{ $match: {"channelName": "ABC"} },
{ $unwind : "$rtData" },
{ $group : {
_id: { $hour: "$rtData.ts" },
ucast: { $sum: { $arrayElemAt: [ "$rtData.data", 0 ] } }
}}
])
If your MongoDB does not support $arrayElemAt ( prior to 3.2 ), then you can instead use $first in an additional $group on just the document key, done before you "accumulate" for the desired grouping key:
db.channels.aggregate( [
{ $match: {"channelName": "ABC"} },
{ $unwind : "$rtData" },
{ $group: {
_id: { _id: "$_id", ts: "$rtData.ts" },
data: { $first: "$rtData.data" }
}},
{ $group : {
_id: { $hour: "$_id.ts" },
ucast: { $sum: "$data" }
}}
])
In modern versions you can "double barrel" the $sum to both add up array elements as well as act as an accumulator if you wanted to "sum" all elements of the array:
db.channels.aggregate( [
{ $match: {"channelName": "ABC"} },
{ $unwind : "$rtData" },
{ $group : {
_id: { $hour: "$rtData.ts" },
ucast: { $sum: { $sum: "$rtData.data" } }
}}
])
And with older versions ( prior to 3.2 ) you would "double" $unwind for each array path instead:
db.channels.aggregate( [
{ $match: {"channelName": "ABC"} },
{ $unwind : "$rtData" },
{ $unwind : "$rtData.data" },
{ $group : {
_id: { $hour: "$rtData.ts" },
ucast: { $sum: "$rtData.data" }
}}
])

Answer 2

you need to use $first operator for that instead of $sum
db.channels.aggregate( [ {$match: {"channelName": "ABC"} }, { $unwind : "$rtData" }, { $group : {_id: { $hour: "$rtData.ts" }, ucast: { $first: $rtData.data} }
which will give you output like { "_id" : 28, "ucast" : [ 676.297664, 676.297664 ] }
if you want output like { "_id" : 28, "ucast" : 676.297664 } in next $project or $addFields stage use $arrayElemAt