Get only the last element of array mongoose - node.js

Question

I have array in a document, and I try to receive the last element of this array.
My code is:
Post.find({_id:postId},{'comments':{'$slice':-1}});
this gives me all the object but the comments array contains only the last element.
on the other hand,
Post.find({_id:postId},{'comments':1});
give me only the comments.
I dont find how to combine the two commands together. How it can be done?
{
"users":[],
"comments":["string1","string2","string3"],
"lastValue":"Wow"
"name":"jow"
"_id": {
"$oid": "5747d6bdecfae9d0560077cc"
},
}
Thanks

Answer 1

You might want to use mongodb (version 3.2) aggregation $slice like that:
Post.aggregate([
{
$match: {
'_id.$oid': postId
}
},
{
$project: {
comments: {
$slice: [ "$comments", -1 ]
}
}
}
]);
In earlier versions of mongodb:
Post.aggregate([
{
$match: {
'_id.$oid': postId
}
},
{
$unwind: "$comments"
},
{
$group : {
_id: "$_id.$oid",
comment: { $last: "$comments" }
}
}
]);

Answer 2

I hope this helps.
db.Post.find(
{ _id: postId },
{ comments: { $slice: -1 }, _id: 0, users: 0, lastValue: 0, name: 0 },
);

Answer 3

In case of Mongoose, slice can work this way also,
model.find({
// condition
})
.select('fields')
.slice('array', -1) // <------ Here
.then((data) => {
// handle
})
.catch();
Just wrote pseudo code, as it might help someone.

Answer 4

db.collection_name.find({'name':'how'},{'comments': {$slice: -1}})