MongoDB: Get the previous and next element in the embedded array - mongodb

Question

I have a database which has the following structure:
{
"_id" : ObjectId("59b8d72ab515211f3c161c4b"),
"Transport_event_id" : 1,
"Carrier_id" : 23,
"Payload_id" : 0,
"StartTime" : 214392.0,
"EndTime" : 362707.0,
"Move_events" : [
{
"Timestamp" : 214398,
"x_pos" : 13,
"y_pos" : 202
},{
"Timestamp" : 214845,
"x_pos" : 12,
"y_pos" : 202
},{
"Timestamp" : 216399,
"x_pos" : 12,
"y_pos" : 216
},{
"Timestamp" : 216842,
"x_pos" : 11,
"y_pos" : 216
},{
"Timestamp" : 219586,
"x_pos" : 10,
"y_pos" : 216
}
]
}
I've made the following query which will return the next 2 Elements form a Array after a specific TimeStamp.
var cursor = db.Transport_eventBeta.aggregate([
{ "$match": { "StartTime": { "$lte": query_time } } },
{ "$match": { "EndTime": { "$gte": query_time } } },
{
"$project": {
"Move_events": {
"$let": {
"vars": {
"filtered": {
"$filter": {
"input": "$Move_events",
"as": "event",
"cond": { "$lte": ["$$event.Timestamp" , query_time] }
}
}
},
"in": {
"$slice": [
"$Move_events",
{"$size": "$$filtered"},
2
]
}
}
},
"Carrier_id": 1
}
}
])
while (cursor.hasNext()) {
print(cursor.next());
}
What I need are the documents befor and after this specific TimeStamp.
Some kind of this:
"$slice": [
"$Move_events",
{"$size": "$$filtered"} - 1,
2
]
But this doesn't work. How can I solve this problem? 2 separate queries are no option because of the duration.

Answer 1

You can try below aggregation query in 3.4.
The query will filter Move_events to keep events with timestamp less than input timestamp followed by $arrayElemAt to get the Move_events after and before event.
db.Transport_eventBeta.aggregatee([
{
"$match": {
"StartTime": {
"$lte": query_time
},
"EndTime": {
"$gte": query_time
}
}
},
{
"$project": {
"Move_events": {
"$let": {
"vars": {
"filtered": {
"$filter": {
"input": "$Move_events",
"as": "event",
"cond": {
"$lte": [
"$$event.Timestamp",
query_time
]
}
}
}
},
"in": [
{
"$arrayElemAt": [
"$Move_events",
{
"$subtract": [
{
"$size": "$$filtered"
},
1
]
}
]
},
{
"$arrayElemAt": [
"$Move_events",
{
"$size": "$$filtered"
}
]
}
]
}
}
}
}
])