How do I aggregate and avg the time between two dates? - mongodb

Question

Assuming the below is my element structure. How can I SHELL query the mongodb and get the avg difference (average length) each trip took for every trip in the db? I am guessing subtracting dates? But then how to subtract and then avg?
"_id": {
"$oid": "5445ab058767000062"
},
"comment": null,
"scheduled_request": false,
"status": "blah",
"timestamp_started": {
"$date": "2014-10-21T00:38:28.990Z"
},
"timestamp_transaction_complete": {
"$date": "2014-10-21T00:49:12.990Z"
},
"user_id": "5445a9000057"
UDPATE ========
Here is my query
db.ambulance_requests.aggregate([
{ "$group": {
"_id": null,
"avg_time": {
"$avg": {
"$subtract": [
"$timestamp_transaction_complete",
"$timestamp_started"
]
}
}
}}
])
AND MY RESULT (from a Mac Terminal Shell):
{ "_id" : null, "avg_time" : 0 }

Answer 1

You $subtract and $avg by applying them in a $group pipeline stage. For "everything", use null for the grouping key:
db.trips.aggregate([
{ "$group": {
"_id": null,
"avg_time": {
"$avg": {
"$subtract": [
{ "$ifNull": [ "$timestamp_completed", 0 ] },
{ "$ifNull": [ "$timestamp_started", 0 ] }
]
}
}
}}
])
When you $subtract on BSON Date object from another, the difference is returned as the milliseconds interval between them. This is also a generally handy technique for extracting the milliseconds value for other purposes.
Your single document as supplied:
{
"comment" : null,
"scheduled_request" : false,
"status" : "blah",
"timestamp_started" : ISODate("2014-10-21T00:38:28.990Z"),
"timestamp_completed" : ISODate("2014-10-21T00:49:12.990Z"),
"user_id" : "5445a9000057"
}
The result from your single document in the question:
/* 1 */
{
"_id" : null,
"avg_time" : 644000.0
}

Answer 2

https://mongoplayground.net/p/nFO54i5GIXU
if finishedAt dose not exist in a doc then skip that document from avg calculation
db.collection.aggregate([
{
"$match": {
"finishedAt": {
"$exists": true
}
}
},
{
"$unwind": "$tags"
},
{
"$match": {
"$or": [
{
"tags.name": "Canada"
},
{
"tags.name": "ABC"
},
]
}
},
{
"$group": {
"_id": null,
"avg_time": {
"$avg": {
"$subtract": [
"$finishedAt",
"$createdAt"
]
}
}
}
}
])