Lookup and aggregate multiple levels of subdocument in Mongodb - mongodb

Question

I've tried many answers to similar problems using $lookup, $unwind, and $match, but I can't get this to work for my sub-sub-subdocument situation.
I have this collection, Things:
{
"_id" : ObjectId("5a7241f7912cfc256468cb27"),
"name" : "Fortress of Solitude",
"alias" : "fortress_of_solitude",
},
{
"_id" : ObjectId("5a7247ec548c9ad042f579e2"),
"name" : "Batcave",
"alias" : "batcave",
},
{
"_id" : ObjectId("6a7247bc548c9ad042f579e8"),
"name" : "Oz",
"alias" : "oz",
},
and this one-document collection, Venues:
{
"_id" : ObjectId("5b9acabbbf71f39223f8de6e"),
"name" : "The Office",
"floors" : [
{
"name" : "1st Floor",
"places" : [
{
"name" : "Front Entrance",
"alias" : "front_entrance"
}
]
},
{
"name" : "2nd Floor",
"places" : [
{
"name" : "Batcave",
"alias" : "batcave"
},
{
"name" : "Oz",
"alias" : "oz"
}
]
}
]
}
I want to return all the Things, but with the Venue's floors.places.name aggregated with each Thing if it exists if the aliases match between Things and Venues. So, I want to return:
{
"_id" : ObjectId("5a7241f7912cfc256468cb27"),
"name" : "Fortress of Solitude",
"alias" : "fortress_of_solitude",
<-- nothing added here because
<-- it's not found in Venues
},
{
"_id" : ObjectId("5a7247ec548c9ad042f579e2"),
"name" : "Batcave",
"alias" : "batcave",
"floors" : [ <-- this should be
{ <-- returned
"places" : [ <-- because
{ <-- the alias
name" : "Batcave" <-- matches
} <-- in Venues
] <--
} <--
] <--
},
{
"_id" : ObjectId("6a7247bc548c9ad042f579e8"),
"name" : "Oz",
"alias" : "oz",
"floors" : [ <-- this should be
{ <-- returned
"places" : [ <-- because
{ <-- the alias
name" : "Oz" <-- matches
} <-- in Venues
] <--
} <--
] <--
}
I've gotten as far as the following query, but it only returns the entire Venues.floors array as an aggregate onto each Thing, which is way too much extraneous data aggregated. I just want to merge each relevant floor.place sub-subsubdocument from Venues into its corresponding Thing if it exists in Venues.
db.getCollection('things').aggregate([
{$lookup: {from: "venues",localField: "alias",foreignField: "floors.places.alias",as: "matches"}},
{
$replaceRoot: { newRoot: { $mergeObjects: [ { $arrayElemAt: [ "$matches", 0 ] }, "$$ROOT" ] } }
},
{ $project: { matches: 0 } }
])
I'm struggling with existing answers, which seem to change at MongoDB version 3.2, 3.4, 3.6, or 4.2 to include or not include $unwind, $pipeline, and other terms. Can someone explain how to get a sub-sub-subdocument aggregated like this? Thanks!

Answer 1

You can try this :
db.things.aggregate([
{
$lookup:
{
from: "venues",
let: { alias: "$alias" },
pipeline: [
{ $unwind: { path: "$floors", preserveNullAndEmptyArrays: true } },
{ $match: { $expr: { $in: ['$$alias', '$floors.places.alias'] } } },
/** Below stages are only if you've docs like doc 2 in Venues */
{ $addFields: { 'floors.places': { $filter: { input: '$floors.places', cond: { $eq: ['$$this.alias', '$$alias'] } } } } },
{ $group: { _id: '$_id', name: { $first: '$name' }, floors: { $push: '$floors' } } },
{$project : {'floors.places.alias': 1, _id :0}} // Optional
],
as: "matches"
}
}
])
Test : MongoDB-Playground

Answer 2

Since MongoDB v3.6, we may perform uncorrelated sub-queries which gives us more flexibility to join two collections.
Try this:
db.things.aggregate([
{
$lookup: {
from: "venues",
let: {
"alias": "$alias"
},
pipeline: [
{
$unwind: "$floors"
},
{
$project: {
_id: 0,
places: {
$filter: {
input: "$floors.places",
cond: {
$eq: [
"$$alias",
"$$this.alias"
]
}
}
}
}
},
{
$match: {
"places.0": {
$exists: true
}
}
},
{
$unset: "places.name"
}
],
as: "floors"
}
}
])
MongoPlayground