how to find unique values from array of object in mongoDB - database

Question

i have following bson data in mongoDB
{name : "c1"
arr : [
{
partyName : "p1",
poNumber : "789",
},
{
partyName : "p1",
poNumber : "700",
},
{
partyName : "p3",
poNumber : "889",
}
]
},
{name : "c2"
arr : [
{
partyName : "p1",
poNumber : "789",
},
{
partyName : "p2",
poNumber : "700",
},
{
partyName : "p3",
poNumber : "889",
}
]
}
i want all unique values of partyName of name: "c1" object like [p1,p3]. i tried this
const unique = await User.distinct({name :"c1","createPurchaseOrder.partyName"})
(note :- User is my schema name )
but it gives error, i tried to search on web but cant find solution please help me with this

Answer 1

One option is using $reduce:
db.collection.aggregate([
{$match: {name: "c1"}},
{$project: {
res: {
$reduce: {
input: "$arr",
initialValue: [],
in: {$setUnion: ["$$value", ["$$this.partyName"]]}
}
}
}
}
])
See how it works on the playground example

Answer 2

Query1
match for the document with name=c1
path to take the array with the partyName and union with the empty array to remove the duplicates
union => order can be lost in patyNames
Playmongo
aggregate(
[{"$match": {"name": {"$eq": "c1"}}},
{"$project":
{"_id": 0, "parties-unique": {"$setUnion": ["$arr.partyName", []]}}}])
Query2
the above is fast and simple but loses the order of the names
if you want to keep the original order, you can use this instead
reduce and add on array only if element doesn't already exists
Playmongo
aggregate(
[{"$match": {"name": {"$eq": "c1"}}},
{"$set":
{"parties-unique":
{"$reduce":
{"input": "$arr.partyName",
"initialValue": [],
"in":
{"$cond":
[{"$in": ["$$this", "$$value"]}, "$$value",
{"$concatArrays": ["$$value", ["$$this"]]}]}}}}}])