easyinsertOne insertMany
You pass an array of documents to insertMany. By default, one document in the middle fails a unique index check. What happens?
db.users.insertMany([doc1, doc2_dup, doc3])MongoDB Interview Questions (589 to practise)
MongoDB is a document database. Its interviews are about data modelling and index design far more than about query syntax.
Embedding versus referencing, multikey and compound index behaviour, aggregation pipeline stages, read and write concern trade-offs, sharding keys, and replica-set elections.
- Total questions
- 589
- Easy
- 99
- Medium
- 246
- Hard
- 244
Where it shows up: Backend roles at document-store shops, analytics pipelines, and teams running Atlas in production.
Sample MongoDB questions
Twenty real MongoDB questions from the library, code snippet included. Nothing here is paraphrased for search engines. It is the same text a signed-in user sees.
easy$push and $pull
Given { _id: 1, items: [{ k: 'a', n: 1 }, { k: 'b', n: 2 }, { k: 'a', n: 3 }] }, what does $pull: { items: { k: 'a' } } do?
db.col.updateOne({ _id: 1 }, { $pull: { items: { k: 'a' } } })easydocuments and collections
Two documents in the same collection look like this. Is this allowed?
db.people.insertOne({ name: "Ann", age: 30 })
db.people.insertOne({ name: "Bob", city: "NYC", hobbies: ["chess"] })easydeleteOne deleteMany
The collection has 50 documents. What does this command do?
db.logs.deleteMany({})easyfind and query filters
What does find() with an empty filter document return?
db.users.find({})easyupdateOne and $set
What does updateOne do when more than one document matches the filter?
db.orders.updateOne(
{ status: "pending" },
{ $set: { status: "shipped" } }
)easy_id and ObjectId
What does this code print?
const ObjectId = require("mongodb").ObjectId;
const a = new ObjectId("507f1f77bcf86cd799439011");
const b = new ObjectId("507f1f77bcf86cd799439011");
console.log(a === b);
console.log(a.equals(b));easyupsert basics
An updateOne with upsert: true matches no existing document. Which fields are written into the newly inserted document?
db.col.updateOne(
{ sku: 'A1' },
{ $set: { qty: 10 } },
{ upsert: true }
)easysort limit skip
Values for score are [50, 90, 70, 20]. Which document's score does this query return first?
db.players.find().sort({ score: -1 }).limit(1)easyprojection include exclude
Given { _id: 1, name: "Ada", age: 30, city: "NYC" }, what is returned?
db.users.find({}, { age: 0, city: 0 })easysort limit skip
Regardless of how you chain these methods in the shell, in what order does MongoDB apply them?
db.users.find().limit(5).sort({ age: 1 }).skip(2)easyarrays in documents
A document has `tags: ["red", "blue", "green"]`. What does this query match?
db.products.find({ tags: "blue" })easyupsert basics
You want to set 'createdAt' only when a document is first inserted via upsert, but never overwrite it on later updates. Which operator does this?
db.col.updateOne(
{ _id: 1 },
{ $set: { seen: true }, $setOnInsert: { createdAt: new Date() } },
{ upsert: true }
)easycomparison operators $gt $in
Given documents with a numeric price field, what does this filter match?
db.products.find({ price: { $gt: 100 } })easydeleteOne deleteMany
When deleteOne finds no document matching the filter, what is returned?
db.users.deleteOne({ email: "missing@example.com" })easy$push and $pull
A document has `nums: [1, 2, 2, 3, 2]`. After running this update, what is `nums`?
db.col.updateOne(
{ _id: 1 },
{ $pull: { nums: 2 } }
)easycountDocuments
You call db.orders.countDocuments({ status: 'shipped' }, { limit: 5 }). The collection has 12 shipped orders. What is returned?
db.orders.countDocuments({ status: 'shipped' }, { limit: 5 })easy$push and $pull
You want to add multiple values to an array in one update. What is wrong with `$push: { tags: ["a", "b"] }`?
db.col.updateOne(
{ _id: 1 },
{ $push: { tags: ["a", "b"] } }
)easyinsertOne insertMany
What does this insertMany call do when run on an empty collection?
db.nums.insertMany([
{ _id: 1, v: "a" },
{ _id: 1, v: "b" },
{ _id: 2, v: "c" }
])easyfind and query filters
What is the type of the value held in `result` after this line runs?
const result = db.users.find({ active: true });Sign in to answer
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MongoDB topics covered
53 distinct MongoDB topics are tagged across the library. These are the real topic labels stored on the questions, not a hand-written list.
- covered queries internals (25)
- WiredTiger storage engine (23)
- multikey array indexes (23)
- aggregation $match $group (23)
- read concern levels (23)
- text and $regex search (20)
- partial and sparse indexes (19)
- multi-document transactions (19)
- embedding vs referencing (19)
- mongos query routing (19)
- bucket and outlier patterns (18)
- Aggregation pipeline execution (17)
- TTL indexes (16)
- the replica set oplog (16)
- $lookup joins (15)
- $unwind arrays (14)
- Working set and memory (14)
- replica-set elections (14)
- $facet multi-pipeline (13)
- chunk splitting and balancing (12)
- documents and collections (12)
- updateOne and $set (12)
- single-field indexes (11)
- compound indexes (11)
- change streams (11)
- Index intersection (11)
- journaling and durability (10)
- snapshot isolation internals (10)
- write concern w majority (10)
- read preference and staleness (10)
- comparison operators $gt $in (9)
- sharding and shard keys (9)
- find and query filters (8)
- geospatial queries (8)
- insertOne insertMany (8)
- schema validation rules (8)
- $push and $pull (7)
- explain basics (7)
- sort limit skip (6)
- query planner plan cache (6)
- deleteOne deleteMany (5)
- the WiredTiger storage engine (5)
- the query planner plan cache (5)
- _id and ObjectId (4)
- projection include exclude (4)
- nested embedded documents (3)
- BSON data types (3)
- upsert basics (3)
- replica set oplog (3)
- arrays in documents (2)
- countDocuments (2)
- text and regex search (2)
- replica set elections (2)
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