Calculate duration based the same field between two documents - mongodb

Question

I have many collections. I have especially two types of collections:1. collections corresponding to the begin of the visualization of content in e-learning plateforme; 2. collections corresponding to the endof the visualization of this content. Here exemple of begin and end visualization collection:
Begin visualization:
"statement" : {
"actor" : {
"objectType" : "Agent",
"name" : "exemple1#gmail.com",
"mbox" : "mailto:exemple1#gmail.com"
},
"verb" : {
"id" : "http://adlnet.gov/expapi/verbs/launched",
"display" : {
"en" : "Launching the next piece of learning content",
}
},
"object" : {
"objectType" : "Activity",
"id" : "https://exemple1.com/activities/plannode=5857",
"definition" : {
"name" : {
"en" : "calculs",
},
"type" : "http://id.tincanapi.com/activitytype/legacy-learning-standar"
}
},
"context" : {
"platform" : "exemple",
"language" : "en",
},
"timestamp" : "2017-01-16T23:45:06+01:00",
"id" : "0d7c852d-f1bf-45be-8a25-aff286bf1c30",
"version" : "1.0.0"
},
End visualization:
"statement" : {
"actor" : {
"objectType" : "Agent",
"name" : "exemple1#gmail.com",
"mbox" : "mailto:exemple1#gmail.com"
},
"verb" : {
"id" : "http://adlnet.gov/expapi/verbs/terminated",
"display" : {
"en" : "Launching the next piece of learning content",
}
},
"object" : {
"objectType" : "Activity",
"id" : "https://exemple1.com/activities/plannode=5857",
"definition" : {
"name" : {
"en" : "calculs",
},
"type" : "http://id.tincanapi.com/activitytype/legacy-learning-standar"
}
},
"context" : {
"platform" : "exemple",
"language" : "en",
},
"timestamp" : "2017-01-16T23:45:06+01:00",
"id" : "0d7c852d-f1bf-45be-8a25-aff286bf1c30",
"version" : "1.0.0"
},
Now, what I want to do is to group theses collection by object.id and calculate the duration of the content visualization = timestamp (content end visualization) - timestamp (content begin visualization).
I try the following code but it does not work:
db.statements.aggregate([
{$match:{'statement.verb.id':{"$in":["http://adlnet.gov/expapi/verbs/launched", "http://adlnet.gov/expapi/verbs/terminated"]}}]},
{$group:{_id:{
objectid: "$statement.object.id",
learner: "$statement.actor.mbox"
},
duration : {$subtract: ["$timestamp", "$timestamp"}]}}
}
}
])
Can any help me to calculate duration between theses two collections (duration need to be caculated based on two fields having the same name :timestamp.

Answer 1

You can use below aggregation.
db.statements.aggregate([
{"$match":{"statement.verb.id":{"$in":["http://adlnet.gov/expapi/verbs/launched","http://adlnet.gov/expapi/verbs/terminated"]}}},
{"$group":{
"_id":{"objectid":"$statement.object.id","learner":"$statement.actor.mbox"},
"first":{"$first":"$timestamp"},"second":{"$last":"$timestamp"}
}},
{"$addFields":{"duration":{"$abs":{"$subtract":["$first","$second"]}}}}
])