MongoDB Query to Find Last N Documents Easily

Use db.collection.find().sort({_id: -1}).limit(n) to get the last n documents based on insertion order in MongoDB.

Examples

InputFind last 2 documents from collection with _id 1,2,3,4

Output[{_id:4, ...}, {_id:3, ...}]

InputFind last 3 documents from collection with timestamps

Output[{_id:5, ...}, {_id:4, ...}, {_id:3, ...}]

InputFind last 5 documents from empty collection

Output[]

How to Think About It

To find the last n documents, think of sorting the documents in reverse order by their unique identifier or timestamp, then picking the first n from that sorted list. This way, you get the most recent entries first.

Algorithm

1

Access the collection you want to query.

2

Sort the documents by the _id field in descending order to get newest first.

3

Limit the result to n documents to get only the last n entries.

4

Return the resulting documents.

Code

mongodb

const n = 3;
const lastDocuments = db.collection.find().sort({_id: -1}).limit(n).toArray();
printjson(lastDocuments);

Output

[
  {"_id": 5, "name": "Eve"},
  {"_id": 4, "name": "Dave"},
  {"_id": 3, "name": "Carol"}
]

Dry Run

Let's trace finding last 3 documents from a collection with _id 1 to 5.

1

Sort documents by _id descending

Documents order after sort: 5, 4, 3, 2, 1

2

Limit to 3 documents

Selected documents: 5, 4, 3

Step	Documents
After sort	[5, 4, 3, 2, 1]
After limit	[5, 4, 3]

Why This Works

Step 1: Sorting by _id descending

Using sort({_id: -1}) orders documents from newest to oldest because MongoDB ObjectIds increase over time.

Step 2: Limiting results

Applying limit(n) restricts the output to only the last n documents after sorting.

Alternative Approaches

Sort by timestamp field

mongodb

const n = 3;
const lastDocs = db.collection.find().sort({createdAt: -1}).limit(n).toArray();
printjson(lastDocs);

Use this if documents have a timestamp field instead of relying on _id.

Use aggregation pipeline

mongodb

const n = 3;
const lastDocs = db.collection.aggregate([
  { $sort: { _id: -1 } },
  { $limit: n }
]).toArray();
printjson(lastDocs);

Aggregation can be more flexible for complex queries but is slightly more verbose.

Complexity: O(n log n) time, O(n) space

Time Complexity

Sorting the documents takes O(n log n) time where n is the number of documents in the collection.

Space Complexity

The query uses O(n) space to hold documents during sorting and limiting.

Which Approach is Fastest?

Sorting by indexed fields like _id is fastest; aggregation adds overhead but offers flexibility.

Approach	Time	Space	Best For
Find + sort + limit	O(n log n)	O(n)	Simple last n documents retrieval
Sort by timestamp field	O(n log n)	O(n)	When timestamp is available and reliable
Aggregation pipeline	O(n log n)	O(n)	Complex queries needing more stages