MongoDB Aggregation - Lookup pipeline not returning any documents - mongodb

Question

I'm having hard time getting $lookup with a pipeline to work in MongoDB Compass.
I have the following collections:
Toys
Data
[
{
"_id": {
"$oid": "5d233c3bb173a546386c59bb"
},
"type": "multiple",
"tags": [
""
],
"searchFields": [
"Jungle Stampers - Two",
""
],
"items": [
{
"$oid": "5d233c3cb173a546386c59bd"
},
{
"$oid": "5d233c3cb173a546386c59be"
},
{
"$oid": "5d233c3cb173a546386c59bf"
},
{
"$oid": "5d233c3cb173a546386c59c0"
},
{
"$oid": "5d233c3cb173a546386c59c1"
},
{
"$oid": "5d233c3cb173a546386c59c2"
},
{
"$oid": "5d233c3cb173a546386c59c3"
},
{
"$oid": "5d233c3cb173a546386c59c4"
}
],
"name": "Jungle Stampers - Two",
"description": "",
"status": "active",
"category": {
"$oid": "5cfe727cac920000086b880e"
},
"subCategory": "Stamp Sets",
"make": "",
"defaultCharge": null,
"defaultOverdue": null,
"sizeCategory": {
"$oid": "5d0cfde57561e107c88fbde3"
},
"ageFrom": {
"$numberInt": "24"
},
"ageTo": {
"$numberInt": "120"
},
"images": [
{
"_id": {
"$oid": "5d233c3bb173a546386c59bc"
},
"id": {
"$oid": "5d233c39b173a546386c59ba"
},
"url": "/toyimages/5d233c39b173a546386c59ba.jpg",
"thumbUrl": "/toyimages/thumbs/tn_5d233c39b173a546386c59ba.jpg"
}
],
"__v": {
"$numberInt": "2"
}
}
]
Loans
Data
[
{
"_id": {
"$oid": "5e1f1661b712215978c746d9"
},
"tags": [],
"member": {
"$oid": "5e17495e4f81ab3f900dbb63"
},
"source": "admin portal - potter1#gmail.com",
"items": [
{
"id": {
"$oid": "5e1f160eb712215978c746d5"
},
"status": "new",
"_id": {
"$oid": "5e1f1661b712215978c746db"
},
"toy": {
"$oid": "5d233c3bb173a546386c59bb"
},
"cost": {
"$numberInt": "0"
}
},
{
"id": {
"$oid": "5e1f160eb712215978c746d5"
},
"status": "new",
"_id": {
"$oid": "5e1f1661b712215978c746da"
},
"toy": {
"$oid": "5d233b1ab173a546386c59b5"
},
"cost": {
"$numberInt": "0"
}
}
],
"dateEntered": {
"$date": {
"$numberLong": "1579095632870"
}
},
"dateDue": {
"$date": {
"$numberLong": "1579651200000"
}
},
"__v": {
"$numberInt": "0"
}
}
]
I am trying to return a list of toys and their associated loans that have a status of 'new' or 'out'.
I can use the following $lookup aggregate to fetch all loans:
{
from: 'loans',
localField: '_id',
foreignField: 'items.toy',
as: 'loansSimple'
}
However I am trying to use a pipeline to load loans that have the two statuses I am interested in, but it always only returns zero documents:
{
from: 'loans',
let: {
'toyid': '$_id'
},
pipeline: [
{
$match: {
$expr: {
$and: [
{$eq: ['$items.toy', '$$toyid']},
{$eq: ['$items.status', 'new']} // changed from $in to $eq for simplicity
]
}
}
}
],
as: 'loans'
}
This always seems to return 0 documents, however I arrange it:
Have I made a mistake somewhere?
I'm using MongoDB Atlas, v4.2.2, MongoDB Compass v 1.20.4

Answer 1

You are trying to search $$toyid inside inner array, but Operator Expression $eq cannot resolve it.
Best solution: $let (returns filtered loans by criteria) + $filter (applies filter for inner array) operator helps us to get desired result.
db.toys.aggregate([
{
$lookup: {
from: "loans",
let: {
"toyid": "$_id",
"toystatus": "new"
},
pipeline: [
{
$match: {
$expr: {
$gt: [
{
$size: {
$let: {
vars: {
item: {
$filter: {
input: "$items",
as: "tmp",
cond: {
$and: [
{
$eq: [
"$$tmp.toy",
"$$toyid"
]
},
{
$eq: [
"$$tmp.status",
"$$toystatus"
]
}
]
}
}
}
},
in: "$$item"
}
}
},
0
]
}
}
}
],
as: "loans"
}
}
])
MongoPlayground
Alternative solution 1. Use $unwind to flatten items attribute. (We create extra field named tmp which stores items value, flatten it with $unwind operator, match as you were doing and then exclude from result)
db.toys.aggregate([
{
$lookup: {
from: "loans",
let: {
"toyid": "$_id"
},
pipeline: [
{
$addFields: {
tmp: "$items"
}
},
{
$unwind: "$tmp"
},
{
$match: {
$expr: {
$and: [
{
$eq: [
"$tmp.toy",
"$$toyid"
]
},
{
$eq: [
"$tmp.status",
"new"
]
}
]
}
}
},
{
$project: {
tmp: 0
}
}
],
as: "loans"
}
}
])
MongoPlayground
Alternative solution 2. We use $reduce to create toy's array and with $in operator we check if toyid exists inside this array.
db.toys.aggregate([
{
$lookup: {
from: "loans",
let: {
"toyid": "$_id"
},
pipeline: [
{
$addFields: {
toys: {
$reduce: {
input: "$items",
initialValue: [],
in: {
$concatArrays: [
"$$value",
[
"$$this.toy"
]
]
}
}
}
}
},
{
$match: {
$expr: {
$in: [
"$$toyid",
"$toys"
]
}
}
},
{
$project: {
toys: 0
}
}
],
as: "loans"
}
}
])

Answer 2

$expr receives aggregation expressions, At that point $$items.toy is parsed for each element in an array as you would expect (however if it would it will still give you "bad" results as you'll get loans that have the required toy id and any other item with status new in their items array).
So you have two options to work around this:
If you don't care about the other items in the lookup'd document you can add an $unwind stage at the start of the lookup pipeline like so:
{
from: 'loans',
let: {
'toyid': '$_id'
},
pipeline: [
{
$unwind: "$items"
},
{
$match: {
$expr: {
$and: [
{$eq: ['$items.toy', '$$toyid']},
{$eq: ['$items.status', 'new']} // changed from $in to $eq for simplicity
]
}
}
}
],
as: 'loans'
}
If you do care about them just iterate the array in one of the possible ways to get a 'correct' match, here is an example using $filter
{
from: 'loads',
let: {
'toyid': '$_id'
},
pipeline: [
{
$addFields: {
temp: {
$filter: {
input: "$items",
as: "item",
cond: {
$and: [
{$eq: ["$$item.toy", "$$toyid"]},
{$eq: ["$$item.status", "new"]}
]
}
}
}
}
}, {$match: {"temp.0": {exists: true}}}
],
as: 'loans'
}