MongoDB aggregate two collections, return additional field as count - mongodb

Question

(See edit below)
I am trying to aggregate data from two separate collections within the same MongoDB database.
The "accounts" collection contains user information (cleansed):
{
_id: ObjectId("5c0d64a4224a2900108c005f"),
"username" : "mike22",
"email" : "mike22#<domain>.com",
"country" : GB,
"created" : ISODate("2018-11-26T23:37:49.051Z")
},
{
_id: ObjectId("5a0d64a4527h2880108c0445"),
"username" : "mike23",
"email" : "mike23#<domain>.com",
"country" : DE,
"created" : ISODate("2018-11-26T23:37:49.051Z")
},
{
_id: ObjectId("5a3334a45zzz2884448c0445"),
"username" : "mike24",
"email" : "mike24#<domain>.com",
"country" : DE,
"created" : ISODate("2018-11-26T23:37:49.051Z")
}
The "devices" collection contains device definitions for all users. A user is likely to have many devices defined in this collection and many users devices are in this collection.
A single device within this collection is defined as follows:
{
"_id" : ObjectId("5c10138c73bbe0001018e415"),
"capabilities" : [
"BrightnessController",
"PowerController"
],
"displayCategories" : [
"LIGHT"
],
"friendlyName" : "Test1",
"description" : "Test device 1",
"reportState" : true,
"username" : "mike22",
"endpointId" : 11,
"__v" : 0
},
{
"_id" : ObjectId("5c10138c73bbe0001018e415"),
"capabilities" : [
"PowerController"
],
"displayCategories" : [
"SWITCH"
],
"friendlyName" : "Test2",
"description" : "Test device 2",
"reportState" : true,
"username" : "mike23",
"endpointId" : 12,
"__v" : 0
},
{
"_id" : ObjectId("5c10138c73bbe0001018e415"),
"capabilities" : [
"PowerController"
],
"displayCategories" : [
"SMARTPLUG"
],
"friendlyName" : "Test3",
"description" : "Test device 3",
"reportState" : true,
"username" : "mike22",
"endpointId" : 13,
"__v" : 0
}
I'm able to use the aggregate below to show me a count of device per-user:
db.accounts.aggregate([
{
$lookup: {
from : "devices",
localField : "username",
foreignField : "username",
as : "userdevs"
},
},
{ $unwind:"$userdevs" },
{ $group : { _id : "$username", count : { $sum : 1 } } }
])
Example output from the data/ aggregate above:
{ "_id" : "mike22", "count" : 2 },
{ "_id" : "mike23", "count" : 1 }
(Note user with no devices is now missing/ should be there with a zero count?!)
However, I want to return all fields for each user plus a new field which shows me the count of devices they have in the "devices" collection. The output I am looking for is as below:
{
"_id" : ObjectId("5c0d64a4224a2900108c005f"),
"username" : "mike22",
"email" : "mike22#<domain>.com",
"country" : GB,
"created" : ISODate("2018-11-26T23:37:49.051Z"),
"countDevices": 2
},
{
"_id" : ObjectId("5a0d64a4527h2880108c0445"),
"username" : "mike23",
"email" : "mike23#<domain>.com",
"country" : DE,
"created" : ISODate("2018-11-26T23:37:49.051Z"),
"countDevices": 1
},
{
"_id" : ObjectId("5a0d64a4527h2880108c0445"),
"username" : "mike24",
"email" : "mike24#<domain>.com",
"country" : DE,
"created" : ISODate("2018-11-26T23:37:49.051Z"),
"countDevices": 0
}
Edit 16/12: So I am nearly there with the aggregate below. Zero-count users are missing though.
use users
db.accounts.aggregate([
{
$lookup: {
from : "devices",
localField : "username",
foreignField : "username",
as : "userdevs"
},
},
{ $unwind: "$userdevs"},
{ $group : { _id : {
_id: "$_id",
username: "$username",
email: "$email",
country: "$country",
region: "$region",
},
countDevices : { $sum : 1 } } }
])
2nd Edit 16/12:
I have found the aggregate needed below:
db.accounts.aggregate([
{ "$lookup": {
"from": "devices",
"let": { "username": "$username" },
"pipeline": [
{ "$match": {
"$expr": { "$eq": [ "$$username", "$username" ] }
}},
{ "$count": "count" }
],
"as": "deviceCount"
}},
{ "$addFields": {
"countDevices": { "$sum": "$deviceCount.count" }
}}
])

Answer 1

First of All, you can flatten the answer you have got with a projection like below:
{ $project : {
_id : '$_id._id',
username : '$_id.username',
email : '$_id.email',
country : '$_id.country',
region : '$_id.region',
countDevices: 1
}
}
add this after the $group in your pipeline, you will get your result as you wanted in the question.
About zero-count users, there is a way to handle this in database using mongoDB, as explained in detail here but I do not recommend it, its better that you handle this kind of problem client side.

Answer 2

As-per second edit, the aggregate I used is as below:
db.accounts.aggregate([
{ "$lookup": {
"from": "devices",
"let": { "username": "$username" },
"pipeline": [
{ "$match": {
"$expr": { "$eq": [ "$$username", "$username" ] }
}},
{ "$count": "count" }
],
"as": "deviceCount"
}},
{ "$addFields": {
"countDevices": { "$sum": "$deviceCount.count" }
}}
])