Aggregation error: $arrayElemAt's first argument must be an array, but is object - mongodb

Question

I'm trying to aggregate a collection in mongo using the following pipeline:
const results = await Price.aggregate([
{ $match: { date: today } },
{ $unwind: '$points' },
{ $match: { 'points.time': { $gte: start, $lte: now } } },
{ $sort: { 'points.time': 1 } },
{ $project: {
'high': { $max: '$points.price' },
'low': { $min: '$points.price' },
'open': { $arrayElemAt: ['$points', 0] },
'close': { $arrayElemAt: ['$points', -1] }
} }
])
However the $arrayElemAt operator isn't working preseumably because one of the preceding stages ($unwind I believe) converts the array of points I have in my documents to an object. How can I fix this?
Example document:
{
"_id" : ObjectId("5c93ac3ab89045027259a23f"),
"date" : ISODate("2019-03-21T00:00:00Z"),
"symbol" : "CC6P",
"points" : [
{
"_id" : ObjectId("5c93ac3ab89045027259a244"),
"volume" : 553,
"time" : ISODate("2019-03-21T09:35:34.239Z"),
"price" : 71
},
{
"_id" : ObjectId("5c93ac3ab89045027259a243"),
"volume" : 1736,
"time" : ISODate("2019-03-21T09:57:34.239Z"),
"price" : 49
},
....
],
My expected result is an array of objects where the points that should be passed to the project stage should be points in the specified range in the second $match. I tried combining the two $match stages and removing the $unwind stage and the error is gone however the time range isn't being applied

Answer 1

I believe you are missing a $group stage to rollback your points array
const results = await Price.aggregate([
{ "$match": { "date": today } },
{ "$unwind": "$points" },
{ "$match": { "points.time": { "$gte": start, "$lte": now } } },
{ "$sort": { "points.time": 1 } },
{ "$group": {
"_id": "$_id",
"points": { "$push": "$points" },
"date": { "$first": "$date" },
"symbol": { "$first": "$symbol" }
}},
{ "$project": {
"high": { "$max": "$points.price" },
"low": { "$min": "$points.price" },
"open": { "$arrayElemAt": ["$points", 0] },
"close": { "$arrayElemAt": ["$points", -1] }
}}
])