Returning a document with two fields from the same array in MongoDB - mongodb

Question

Given documents such as
{
_id: 'abcd',
userId: '12345',
activities: [
{ status: 'login', timestamp: '10000001' },
{ status: 'logout', timestamp: '10000002' },
{ status: 'login', timestamp: '10000003' },
{ status: 'logout', timestamp: '10000004' },
]
}
I am trying to create a pipeline such as all users that have their latest login/logout activities recorded between two timestamps will be returned. For example, if the two timestamp values are between 10000002 and 10000003, the expected document should be
{
_id: 'abcd',
userId: '12345',
login: '10000003',
logout: '10000002'
}
Of if the two timestamp values are between -1 and 10000001, the expected document should be :
{
_id: 'abcd',
userId: '12345',
login: '10000001',
logout: null
}
Etc.
I know it has to do with aggregations, and I need to $unwind, etc., but I'm not sure about the rest, namely evaluating two fields from the same document array

Answer 1

You can try below aggregation:
db.col.aggregate([
{
$unwind: "$activities"
},
{
$match: {
$and: [
{ "activities.timestamp": { $gte: "10000001" } },
{ "activities.timestamp": { $lte: "10000002" } }
]
}
},
{
$sort: {
"activities.timestamp": -1
}
},
{
$group: {
_id: "$_id",
userId: { $first: "$userId" },
activities: { $push: "$activities" }
}
},
{
$addFields: {
login: { $arrayElemAt: [ { $filter: { input: "$activities", as: "a", cond: { $eq: [ "$$a.status", "login" ] } } } , 0 ] },
logout: { $arrayElemAt: [ { $filter: { input: "$activities", as: "a", cond: { $eq: [ "$$a.status", "logout" ] } } } , 0 ] }
}
},
{
$project: {
_id: 1,
userId: 1,
login: { $ifNull: [ "$login.timestamp", null ] },
logout: { $ifNull: [ "$logout.timestamp", null ] }
}
}
])
We need to use $unwind + $sort + $group to make sure that our activities will be sorted by timestamp. After $unwind you can use $match to apply filtering condition. Then you can use $filter with $arrayElemAt to get first (latest) value of filtered array. In the last $project you can explicitly use $ifNull (otherwise JSON key will be skipped if there's no value)

Answer 2

You can use below aggregation
Instead of $unwind use $lte and $gte with the $fitler aggregation.
db.collection.aggregate([
{ "$project": {
"userId": 1,
"login": {
"$max": {
"$filter": {
"input": "$activities",
"cond": {
"$and": [
{ "$gte": ["$$this.timestamp", "10000001"] },
{ "$lte": ["$$this.timestamp", "10000004"] },
{ "$lte": ["$$this.status", "login"] }
]
}
}
}
},
"logout": {
"$max": {
"$filter": {
"input": "$activities",
"cond": {
"$and": [
{ "$gte": ["$$this.timestamp", "10000001"] },
{ "$lte": ["$$this.timestamp", "10000004"] },
{ "$lte": ["$$this.status", "logout"] }
]
}
}
}
}
}}
])