MongoDB $sum and $avg of sub documents - mongodb

Question

I need to get $sum and $avg of subdocuments, i would like to get $sum and $avg of Channels[0].. and other channels as well.
my data structure looks like this
{
_id : ... Location : 1,
Channels : [
{ _id: ...,
Value: 25
},
{
_id: ... ,
Value: 39
},
{
_id: ..,
Value: 12
}
]
}

Answer 1

In order to get the sum and average of the Channels.Value elements for each document in your collection you will need to use mongodb's Aggregation processing. Further, since Channels is an array you will need to use the $unwind operator to deconstruct the array.
Assuming that your collection is called example, here's how you could get both the document sum and average of the Channels.Values:
db.example.aggregate( [
{
"$unwind" : "$Channels"
},
{
"$group" : {
"_id" : "$_id",
"documentSum" : { "$sum" : "$Channels.Value" },
"documentAvg" : { "$avg" : "$Channels.Value" }
}
}
] )
The output from your post's data would be:
{
"_id" : SomeObjectIdValue,
"documentSum" : 76,
"documentAvg" : 25.333333333333332
}
If you have more than one document in your collection then you will see a result row for each document containing a Channels array.

Answer 2

Solution 1: Using two groups based this example:
previous question
db.records.aggregate(
[
{ $unwind: "$Channels" },
{ $group: {
_id: {
"loc" : "$Location",
"cId" : "$Channels.Id"
},
"value" : {$sum : "$Channels.Value" },
"average" : {$avg : "$Channels.Value"},
"maximun" : {$max : "$Channels.Value"},
"minimum" : {$min : "$Channels.Value"}
}},
{ $group: {
_id : "$_id.loc",
"ChannelsSumary" : { $push :
{ "channelId" : '$_id.cId',
"value" :'$value',
"average" : '$average',
"maximun" : '$maximun',
"minimum" : '$minimum'
}}
}
}
]
)
Solution 2:
there is property i didn't show on my original question that might of help "Channels.Id" independent from "Channels._Id"
db.records.aggregate( [
{
"$unwind" : "$Channels"
},
{
"$group" : {
"_id" : "$Channels.Id",
"documentSum" : { "$sum" : "$Channels.Value" },
"documentAvg" : { "$avg" : "$Channels.Value" }
}
}
] )