Get the size of all the documents in a query - mongodb

Question

Is there a way to get the size of all the documents that meets a certain query in the MongoDB shell?
I'm creating a tool that will use mongodump (see here) with the query option to dump specific data on an external media device. However, I would like to see if all the documents will fit in the external media device before starting the dump. That's why I would like to get the size of all the documents that meet the query.
I am aware of the Object.bsonsize method described here, but it seems that it only returns the size of one document.

Answer 1

Here's the answer that I've found:
var cursor = db.collection.find(...); //Add your query here.
var size = 0;
cursor.forEach(
function(doc){
size += Object.bsonsize(doc)
}
);
print(size);
Should output the size in bytes of the documents pretty accurately.
I've ran the command twice. The first time, there were 141 215 documents which, once dumped, had a total of about 108 mb. The difference between the output of the command and the size on disk was of 787 bytes.
The second time I ran the command, there were 35 914 179 documents which, once dumped, had a total of about 57.8 gb. This time, I had the exact same size between the command and the real size on disk.

Answer 2

Starting in Mongo 4.4, $bsonSize returns the size in bytes of a given document when encoded as BSON.
Thus, in order to sum the bson size of all documents matching your query:
// { d: [1, 2, 3, 4, 5] }
// { a: 1, b: "hello" }
// { c: 1000, a: "world" }
db.collection.aggregate([
{ $group: {
_id: null,
size: { $sum: { $bsonSize: "$$ROOT" } }
}}
])
// { "_id" : null, "size" : 177 }
This $groups all matching items together and $sums grouped documents' $bsonSize.
$$ROOT represents the current document from which we get the bsonsize.