MongoDB query elements that are in an array - node.js

Question

I am working in a mongoDB + node (with express and mongoose) API.
I have a document with more or less this structure:
// npcs
{
"_id" : ObjectId("5ea6c0f88e8ecfd3cdc39eae"),
"flavor" : {
"gender" : "...",
"description" : "...",
"imageUrl" : "...",
"class" : "...",
"campaign" : [
{
"campaignId" : "5eac9dfe8e8ecfd3cdc41aa0",
"unlocked" : true
}
]
},
},
// ...
And a second document in a separate table that is as follows:
// user
{
"_id" : ObjectId("5e987f8e4b88382a98c84042"),
"username" : "KuluGary",
"campaigns" : [
"5eac9dfe8e8ecfd3cdc41aa0",
"5eac9e458e8ecfd3cdc41ac1",
"5eac9e978e8ecfd3cdc41adb",
"5eac9eae8e8ecfd3cdc41ae3"
]
}
What I want to do is make a query in which I obtain all the NPCs that are a part of a campaign the user is part of, and are unlocked. The second part is fairly easy, just thought of once I retrieve the NPCs to filter those with unclocked false, but I'm having a hard time visualizing the query since I'm fairly unfamiliar with mongoDBs syntax and usage.
Any help would be greatly appreciated.

Answer 1

I understand you want to "join" a user with all relevant NPC's?
A simple aggregation with $lookup would work:
db.userCollection.aggregate([
{
$match: {
// match relevant users with whatever condition you want
}
},
{
$lookup: {
from: "npc_collection",
let: {campaigns: "$campaigns"},
pipeline: [
{
$match: {
$expr: {
$gt: [
{
$size: {
$filter: {
input: "$flavor.campaign",
as: "campaign",
cond: {
$and: [
{$setIsSubset: ["$flavor.campaign.campaignId", "$$campaigns"]},
{$eq: ["$$campaign.unlocked", true]}
]
}
}
}
},
0
]
}
}
}
],
as: "relevant_npcs"
}
}
])
Note that due to the need of an NPC to be active in a specific campaign and not just a unlocked in any we require the use of $filter.
I recommend that if you only want to lookup on one user you split this into 2 calls as i feel using $elemMatch would give better performance:
let campaigns = await db.userCollection.distinct("campaigns", {_id: userId})
let results = await db.npcCollection.find({"flavor.campaign": {$elemMatch: { campaignId: {$in: campaigns}, unlocked: true}}})