Expand Contained Mongo Document - mongodb

Question

This is for MongoDB
I have a Document say Employee as below:
{
"_id":"e01",
"name":"Andy",
"salary":"10000",
"address":"ad01id"
}
That is a simple Employee Document, with address being a foreign key to Address Document as below(for this above Employee)
{
"_id": "ad01id",
"pin":"999",
"Home":{
"street":"101",
"city":"someCity"
},
"Country":"someCountry"
}
I need a help to write a query which will fetch an Employee but will give me an output, which will have Employee's address field expanded with its entire document as
{
"_id":"e01",
"name":"Andy",
"salary":"10000",
"address":{
"pin":"999",
"Home":{
"street":"101",
"city":"someCity"
},
"Country":"someCountry"
}
}
I have tried $lookup and $merge but could not get the desired output - $merge hides the keys in the early document if the later document has the same key, which is the case in my real scenario. My actual documents have some keys with the same name. Such as name is there in both of my actual Documents.
Thanks in advance!

Answer 1

$lookup is needed to "join" the data from both collections and $unwind or $arrayElemAt can be used to convert an array being a result of $lookup into nested subdocument:
db.Employee.aggregate([
{
$lookup: {
from: "Address",
localField: "address",
foreignField: "_id",
as: "address"
}
},
{
$addFields: {
address: { $arrayElemAt: [ "$address", 0 ] }
}
},
{
$project: {
"address._id": 0
}
}
])
Mongo Playground

Answer 2

You can also use populate.
db.Employee.find(
{
},
)
.populate("address", "pin Home Country")
.exec(function(err, result) {
console.log(result)
});