how to get parent with child entries in mongo shell - mongodb

Question

I am new to MongoDB.
my collections are authors and books. its many to many associations
my authors collections
[{
"id": "11",
"name": "Sam",
"age": 40
},
{
"id": "12",
"name": "Jack",
"age": 20
},
{
"id": "22",
"name": "Smith",
"age": 35
}]
my books collections
[{
"id": "123",
"title": "Hello World",
"price": 200,
"authors":["Sam","Jack"]
},
{
"id": "34",
"title": "Good Day",
"price": 100,
"authors":["Smith"]
}]
This is my postman result.
[{
"authors": [
{
"id": "11",
"name": "Sam",
"age": 40,
},
{
"id": "12",
"name": "Jack",
"age": 20,
}
],
"id": "123",
"title": "Hello World",
"price": 200,
}]
I put query in mongo shell to find books but result only shown book details not shown authors(child entries).
how to I get books with authors entries in mongo shell command(like postman result).
Thanks.

Answer 1

Try the following code:
<db>.books.aggregate([
{
'$project': {
'id': 1,
'title': 1,
'price': 1,
'authors': 1,
'numberOfAuthors': {
'$cond': {
'if': {
'$isArray': '$authors'
},
'then': {
'$size': '$authors'
},
'else': 0
}
}
}
}, {
'$match': {
'numberOfAuthors': {
'$eq': 2
}
}
}, {
'$lookup': {
'from': 'authors',
'localField': 'authors',
'foreignField': 'name',
'as': 'authors'
}
}
])
Used projection followed by match before lookup.