MongoDb - MongoDB collections

1. What is a Collection?

• In MongoDB, a collection is the equivalent of a table in relational databases.

• A collection holds documents (JSON-like objects), but unlike SQL tables:

  • Documents in a collection don’t need to have the same fields.

  • The schema is flexible (schema-less by default).

• Collections live inside databases.

Example:

• Database: myDatabase

• Collection: users

• Documents:

  { "name": "Alice", "age": 25 }
  { "username": "bob123", "email": "[email protected]" }
  

Notice: The documents don’t need identical fields.

2. Creating a Collection

a) Explicitly

db.createCollection("users")

You can also pass options:

db.createCollection("logs", {
  capped: true,      // fixed size collection
  size: 5242880,     // size in bytes (5 MB)
  max: 5000          // maximum number of documents
})

b) Implicitly

MongoDB automatically creates a collection the first time you insert a document:

db.products.insertOne({ name: "Laptop", price: 1200 })

This creates products collection if it doesn’t exist yet.

3. Basic Operations on Collections

a) Insert Documents

db.users.insertOne({ name: "Alice", age: 25 })
db.users.insertMany([
  { name: "Bob", age: 30 },
  { name: "Charlie", age: 22 }
])

b) Query Documents

db.users.find({ age: { $gt: 25 } })
db.users.findOne({ name: "Alice" })

c) Update Documents

db.users.updateOne(
  { name: "Alice" },
  { $set: { age: 26 } }
)

db.users.updateMany(
  { age: { $lt: 25 } },
  { $set: { status: "young" } }
)

d) Delete Documents

db.users.deleteOne({ name: "Charlie" })
db.users.deleteMany({ inactive: true })

4. Viewing Collections

List all collections in the current database:

show collections

or:

db.getCollectionNames()

5. Dropping a Collection

Remove a collection completely:

db.users.drop()

6. Special Types of Collections

• Capped collections → fixed size, old docs automatically overwritten (useful for logs).

• Time-series collections → optimized for time-based data (IoT, events, metrics).

• View collections → like SQL views, based on aggregation pipelines.

Example (time-series):

db.createCollection("sensorData", {
  timeseries: { timeField: "timestamp", metaField: "sensorId" }
})