MongoDB Cheat Sheet 2026

The 30 highest-yield MongoDB facts, distilled from real exam questions. Print it, save it as a PDF, or study it here — free, no sign-up.

1. What problem does the Computed Pattern solve in MongoDB? → It pre-computes and stores results that are expensive to calculate on every read
2. Which operator selects documents where a field value matches any value in a specified array? → $in
3. What is horizontal scaling in MongoDB achieved through? → Sharding data across multiple servers
4. How many votes are needed for a primary election in a MongoDB replica set? → A simple majority of all voting members
5. Which write concern provides the lowest durability but the highest write throughput in MongoDB? → { w: 0 }
6. What type of database is MongoDB? → Document Oriented
7. What is the 'outlier pattern' in MongoDB data modeling used for? → Handling documents whose arrays would exceed the size limit by using overflow documents
8. What is a partial index in MongoDB? → An index that only includes documents meeting a specified filter expression
9. What does the mongotop tool monitor in MongoDB? → Time spent reading and writing data per collection
10. Which field is automatically created by MongoDB for every document if not explicitly provided? → _id
11. What is the correct MongoDB syntax to sort query results in descending order by a field named 'score'? → .sort({score: -1})
12. What is the Atlas connection string format used by application drivers? → mongodb+srv://username:password@cluster.mongodb.net/dbname
13. In a wrapped query, ______ is a useful option that sets the maximum number of documents to verify for a given query. → maxscan
14. The command to get a backup of a point-in-time data view is ____. → fsync
15. What is the purpose of the mongoexport tool? → Exports MongoDB data to a CSV or JSON text format
16. Which of the following is a recommended characteristic of a good shard key? → High cardinality and even distribution of values to prevent hot spots
17. How do you apply a projection in a MongoDB find() query to return only specific fields? → db.col.find({filter}, {field: 1})
18. What is an Atlas cluster tier (M0, M10, M30, etc.)? → A predefined configuration defining CPU, RAM, and storage resources for the cluster
19. What is MongoDB's Ops Manager used for? → On-premises deployment management, monitoring, backup, and automation for MongoDB
20. What is zone sharding (also called tag-aware sharding) in MongoDB? → Associating ranges of shard key values with specific shards to control data placement
21. What is the Tree Pattern used for in MongoDB? → Representing hierarchical data such as organizational charts or category trees
22. Which MongoDB feature enforces that fields meet specific data type requirements? → Schema validation with $jsonSchema
23. What does the replaceOne() method do differently from updateOne()? → It replaces the entire document except _id rather than updating specific fields
24. What does mongos do in a MongoDB sharded cluster? → Acts as a query router, directing client operations to the appropriate shards
25. What happens to indexes when a MongoDB collection is dropped? → All indexes on the collection are automatically deleted
26. What is a hashed index used for in MongoDB? → Enabling hash-based sharding by hashing field values
27. What happens to a replica set member whose oplog window is exceeded by replication lag? → It enters RECOVERING state and must be resynced from another member
28. In JavaScript, the data structure representation of a document is ____ . → Object
29. What is a write conflict in a MongoDB transaction? → When two concurrent transactions attempt to modify the same document simultaneously
30. Which aggregation stage writes the results of the pipeline to a specified collection? → $out