Match two equal fields of arrays of same documents without $unwind - node.js

Question

I have below workouts collection
[
{
"receiverWorkout": {
"exercises": [
{ "_id": "1", "reps": 0 },
{ "_id": "2", "reps": 4 }
]
},
"senderWorkout": {
"exercises": [
{ "_id": "2", "reps": 12 },
{ "_id": "1", "reps": 8 }
]
}
}
]
I need to match same _id from receiverWorkout and senderWorkout exercises and want to add a key won true or false whose reps are greater.
If you can see first element of receiverWorkout exercises({ "_id": "1", "reps": 0 }) is matching with the second element of the senderWorkout exercises({ "_id": "1", "reps": 8 }) because both have same _id
Expected output should be something like
[
{
"receiverWorkout": {
"exercises": [
{ "_id": "1", "reps": 0, "won": false },
{ "_id": "2", "reps": 4, "won": false }
]
},
"senderWorkout": {
"exercises": [
{ "_id": "2", "reps": 12, "won": true },
{ "_id": "1", "reps": 8, "won": true }
]
}
}
]
I want to avoid $unwind
Please help!!!

Answer 1

You can use below aggregation.
Single Pass
db.colname.aggregate([
{"$project":{
"exercises":{
"$reduce":{
"input":"$receiverWorkout.exercises",
"initialValue":{"receiverWorkout":[],"senderWorkout":[]},
"in":{"$let":{
"vars":{
"sen":{"$arrayElemAt":["$senderWorkout.exercises",{"$indexOfArray":["$senderWorkout.exercises._id","$$this._id"]}]}
},
"in":{
"receiverWorkout":{
"$concatArrays":[
"$$value.receiverWorkout",
[{"_id":"$$this._id","reps":"$$this.reps","won":{"$gt":["$$this.reps","$$sen.reps"]}}]
]
},
"senderWorkout":{
"$concatArrays":[
"$$value.senderWorkout",
[{"_id":"$$sen._id","reps":"$$sen.reps","won":{"$gt":["$$sen.reps","$$this.reps"]}}]
]
}
}
}}
}
}
}},
{"$project":{
"receiverWorkout.exercises":"$exercises.receiverWorkout",
"senderWorkout.exercises":"$exercises.senderWorkout"
}}
])
Without using $unwind
db.colname.aggregate(
{"$project":{
"receiverWorkout.exercises":{
"$map":{
"input":"$receiverWorkout.exercises",
"in":{
"$let":{
"vars":{"sen":{"$arrayElemAt":["$senderWorkout.exercises",{"$indexOfArray":["$senderWorkout.exercises._id","$$this._id"]}]}},
"in":{
"_id":"$$this._id",
"reps":"$$this.reps",
"won":{"$gt":["$$this.reps","$$sen.reps"]}
}
}
}
}
},
"senderWorkout.exercises":{
"$map":{
"input":"$senderWorkout.exercises",
"in":{
"$let":{
"vars":{"rec":{"$arrayElemAt":["$receiverWorkout.exercises",{"$indexOfArray":["$receiverWorkout.exercises._id","$$this._id"]}]}},
"in":{
"_id":"$$this._id",
"reps":"$$this.reps",
"won":{"$gt":["$$this.reps","$$rec.reps"]}
}
}
}
}
}
}})
With $unwind
db.colname.aggregate([
{"$unwind":"$senderWorkout.exercises"},
{"$unwind":"$receiverWorkout.exercises"},
{"$sort":{"receiverWorkout.exercises._id":1,"senderWorkout.exercises._id":1}},
{"$addFields":{
"receiverWorkout.exercises.won":{
"$cond":[
{"$gt":["$receiverWorkout.exercises.reps","$senderWorkout.exercises.reps"]},
true,
false
]
},
"senderWorkout.exercises.won":{
"$cond":[
{"$gt":["$receiverWorkout.exercises.reps","$senderWorkout.exercises.reps"]},
false,
true
]
}
}},
{"$group":{
"_id":"$_id",
"receiverWorkout":{"$addToSet":"$receiverWorkout.exercises"},
"senderWorkout":{"$addToSet":"$senderWorkout.exercises"}
}}
])

Answer 2

You don't need to use array index to achieve this. If I good understand, you need to know, for each entry in your senderWorkout.exercises array, if an element with the same _id is present in receiverWorkout.exercises array. Here's the way to do this :
db.collection.aggregate([
{
$addFields: {
"senderWorkout.exercises": {
$map: {
input: "$senderWorkout.exercises",
as: "ex",
in: {
$cond: {
if: {
$in: [
"$$ex._id",
"$receiverWorkout.exercises._id"
]
},
then: {
_id: "$$ex._id",
reps: "$$ex.reps",
result: true
},
else: {
_id: "$$ex._id",
reps: "$$ex.reps",
result: false
}
}
}
}
}
}
}
])
you can try it here