Removing duplicates in mongodb with aggregate query - mongodb

Question

db.games.aggregate([
{ $unwind : "$rounds"},
{ $match: {
"rounds.round_values.gameStage": "River",
"rounds.round_values.decision": "BetPlus" }
},
{ $project: {"FinalFundsChange":1, "GameID":1}
}])
The resulting output is:
{ "_id" : ObjectId("57cbce66e281af12e4d0731f"), "GameID" : "229327202", "FinalFundsChange" : 0.8199999999999998 }
{ "_id" : ObjectId("57cbe2fce281af0f34020901"), "FinalFundsChange" : -0.1599999999999997, "GameID" : "755030199" }
{ "_id" : ObjectId("57cbea3ae281af0f340209bc"), "FinalFundsChange" : 0.10000000000000009, "GameID" : "231534683" }
{ "_id" : ObjectId("57cbee43e281af0f34020a25"), "FinalFundsChange" : 1.7000000000000002, "GameID" : "509975754" }
{ "_id" : ObjectId("57cbee43e281af0f34020a25"), "FinalFundsChange" : 1.7000000000000002, "GameID" : "509975754" }
As you can see the last element is a duplicate, that's because the unwind creates two elements of it, which it should. How can I (while keeping the aggregate structure of the query) keep the first element of the duplicate or keep the last element of the duplicate only?
I have seen that the ways to do it seem to be related to either $addToSet or $setUnion (any details how this works exactly are appreciated as well), but I don't understand how I can choose the 'subset' by which I want to identify the duplicates (in my case that's the 'GameID', other values are allowed to be different) and how I can select whether I want the first or the last element.

Answer 1

You could group by _id via $group and then use the $last and $first operator respectively to keep the last or first values.
db.games.aggregate([
{ $unwind : "$rounds"},
{ $match: {
"rounds.round_values.gameStage": "River",
"rounds.round_values.decision": "BetPlus" }
},
{ $group: {
_id: "$_id",
"FinalFundsChange": { $first: "$FinalFundsChange" },
"GameID": { $last: "$GameID" }
}
}
])

Answer 2

My problem was find all users who purchase same product, where a user can purchase a product multiple time.
https://mongoplayground.net/p/UTuT4e_N6gn
db.payments.aggregate([
{
"$lookup": {
"from": "user",
"localField": "user",
"foreignField": "_id",
"as": "user_docs"
}
},
{
"$unwind": "$user_docs",
},
{
"$group": {
"_id": "$user_docs._id",
"name": {
"$first": "$user_docs.name"
},
}
},
{
"$project": {
"_id": 0,
"id": "$_id",
"name": "$name"
}
}
])