Nested conditional MongoDB query - mongodb

Question

Im having a hard time trying to run some nested queries with a conditional statement of an item inside an array.
this is how my documents looks like.
I would like to get a summary such as sum and average and alarmedCount (count every time Channels.AlarmStatus == "alarmed") of each "Channel" based on Channels.Id. I got sum and average to work but cant get the right query for alarmedCount
{
"_id" : "55df8e4cd8afa4ccer1915ee"
"location" : "1",
"Channels" : [{
"_id" : "55df8e4cdsafa4cc0d1915r1",
"ChannelId" : 1,
"Value" : 14,
"AlarmStatus" : "normal"
},
{
"_id" : "55df8e4cdsafa4cc0d1915r9",
"ChannelId" : 2,
"Value" : 20,
"AlarmStatus" : "alarmed"
},
{
"_id" : "55df8e4cdsafa4cc0d1915re",
"ChannelId" : 3,
"Value" : 10,
"AlarmStatus" : "alarmed"},
]
}
{
"_id" : "55df8e4cd8afa4ccer1915e0"
"location" : "1",
"Channels" : [{
"_id" : "55df8e4cdsafa4cc0d19159",
"ChannelId" : 1,
"Value" : 50,
"AlarmStatus" : "normal"
},
{
"_id" : "55df8e4cdsafa4cc0d1915re",
"ChannelId" : 2,
"Value" : 16,
"AlarmStatus" : "normal"
},
{
"_id" : "55df8e4cdsafa4cc0d1915g7",
"ChannelId" : 3,
"Value" : 9,
"AlarmStatus" : "alarmed"},
]
}
I got it to work to group them and show some calculations
using this aggregate
db.records.aggregate( [
{
"$unwind" : "$Channels"
},
{
"$group" : {
"_id" : "$Channels.Id",
"documentSum" : { "$sum" : "$Channels.Value" },
"documentAvg" : { "$avg" : "$Channels.Value" }
}
}
] )
the result looks like this:
{
"result" : [
{
"_id" : 1,
"documentSum" : 64,
"documentAvg" : 32
},
{
"_id" : 2,
"documentSum" : 36,
"documentAvg" : 18
},
{
"_id" : 3,
"documentSum" : 19,
"documentAvg" : 9.5
},
],
"ok" : 1.0000000000000000
}
I would like to get this type of result
{
"result" : [
{
"_id" : 1,
"documentSum" : 64,
"documentAvg" : 32,
"AlarmedCount" : 0
},
{
"_id" : 2,
"documentSum" : 36,
"documentAvg" : 18,
"AlarmedCount" : 1
},
{
"_id" : 3,
"documentSum" : 19,
"documentAvg" : 9.5,
"AlarmedCount" : 2
}
],
"ok" : 1.0000000000000000
}

Answer 1

Use a project-step before your group-step to convert the field AlarmedStatus to 1 or 0 depending on its value:
$project: {
"Channels.value":"$Channels.Value",
"Channels.AlarmCount":{ $cond: {
if: { $eq: ["$Channels.AlarmedStatus", "alarmed"] },
then: 1,
else: 0 }
}
}
Then sum the newly created field to get the aggregated count:
$group : {
"_id" : "$Channels.Id",
"documentSum" : { "$sum" : "$Channels.Value" },
"documentAvg" : { "$avg" : "$Channels.Value" },
"AlarmCount" : { "$sum" : "$Channels.AlarmCount" }
}