MongoDB query, project nested docs? - mongodb

Question

I was wondering if this is possible, or do I need to use the aggregation pipeline instead?
I have read posts such as this, and intuitively feel like it's possible.
Example of docs:
{
"_id": ObjectID("5143ddf3bcf1bfab37d9c6f"),
"permalink": "btxcacmbqkxbpgtpeero",
"author": "machine",
"title": "Declaration of Independence",
"tags": [
"study",
"law"
],
"comments": [
{
"body": "comment 1",
"email": "email_1#test.com",
"author": "machine_1"
},
{
"body": "comment 2",
"email": "email_2#test.com",
"author": "machine_2"
},
{
"body": "comment 3",
"email": "email_3#test.com",
"author": "machine_3"
},
]
"date": ISODate("2013-03-16T02:50:27.878Z")
}
I am trying to access a particular comment in "comments" by it's index position using dot notation in the projection field, with the following:
db.collection.find({permalink: "btxcacmbqkxbpgtpeero"}, {'comments.0.1.': 1})
Where comments.0 is the first item in the field: the array, and .1 is the second comment in the array.
The result I am getting:
{ "_id" : ObjectID("5143ddf3bcf1bfab37d9c6f"), "comments" : [ { }, { }, { } ] }
If I take away the .1, leaving just comments.0, I get the same result:
{ "_id" : ObjectID("5143ddf3bcf1bfab37d9c6f"), "comments" : [ { }, { }, { } ] }
If I take away the .0, leaving just comments, I get the comments still inside their array:
[
{
"body": "comment 1",
"email": "email_1#test.com",
"author": "machine_1"
},
{
"body": "comment 2",
"email": "email_2#test.com",
"author": "machine_2"
},
{
"body": "comment 3",
"email": "email_3#test.com",
"author": "machine_3"
}
]
Can this be done? If so, how?

Answer 1

without aggregation:
db.collection.find({
permalink:"btxcacmbqkxbpgtpeero"
},
{
comments:{
$slice:[
0,
1
]
}
})
returns
{
"_id":ObjectId("583af24824168f5cc566e1e9"),
"permalink":"btxcacmbqkxbpgtpeero",
"author":"machine",
"title":"Declaration of Independence",
"tags":[
"study",
"law"
],
"comments":[
{
"body":"comment 1",
"email":"email_1#test.com",
"author":"machine_1"
}
]
}
try it online: mongoplayground.net/p/LGbVWPyVkFk
with aggregation:
db.collection.aggregate([
{
$match:{
permalink:"btxcacmbqkxbpgtpeero"
}
},
{
$project:{
comment:{
$arrayElemAt:[
"$comments",
0
]
}
}
}
])
returns
{
"_id":ObjectId("583af24824168f5cc566e1e9"),
"comment":{
"body":"comment 1",
"email":"email_1#test.com",
"author":"machine_1"
}
}