MongoDB Collections

Understanding collections and how to work with them

📚 What are Collections?

Collections in MongoDB are groups of documents, similar to tables in relational databases. They store related data together and are created automatically when you insert your first document into them. 


// Create a collection by inserting data
db.customers.insertOne({ name: "Sarah", age: 28 })

// Collection 'customers' is now created

Collection Basics

Collections are flexible containers for documents. Unlike SQL tables, collections don't enforce a strict schema, allowing documents with different structures to coexist in the same collection.

🎯

Auto Creation

Created when first document inserted 

db.orders.insertOne({})
📋

View Collections

List all collections in database 

show collections
➕

Explicit Creation

Create collection manually 

db.createCollection("users")
🗑️

Drop Collection

Delete entire collection 

db.users.drop()

🔹 Creating Collections Implicitly

The easiest way to create a collection is by inserting a document. MongoDB creates the collection automatically: 

// Switch to database
use storeDB

// Insert document (creates 'products' collection)
db.products.insertOne({
    name: "Laptop",
    price: 999,
    brand: "TechCo"
})

// View all collections
show collections

Output:

products

🔹 Creating Collections Explicitly

You can create an empty collection using the createCollection() method: 

// Create empty collection
db.createCollection("employees")

// Create collection with options
db.createCollection("logs", {
    capped: true,
    size: 5242880,  // 5MB
    max: 5000       // max documents
})

// Verify collection exists
show collections

Output:

employees

logs

🔹 Viewing Collections

Use these commands to see collections in your current database: 

// Show all collections
show collections

// Alternative command
show tables

// Get collection names as array
db.getCollectionNames()

Output:

[ "customers", "orders", "products" ]

🔹 Collection Information

Get detailed information about a collection: 

// Get collection statistics
db.customers.stats()

// Count documents in collection
db.customers.countDocuments()

// Check if collection exists
db.getCollectionNames().includes("customers")

Output:

Document count: 150

Collection exists: true

🔹 Dropping Collections

Remove a collection and all its documents permanently: 

// Drop a collection
db.oldData.drop()

// Verify it's gone
show collections

Output:

⚠️ Collection 'oldData' dropped

Warning:

Dropping a collection is permanent and cannot be undone. All documents in the collection will be deleted.

🔹 Collection Naming Rules

Follow these guidelines when naming collections:

  • Start with letter or underscore: Not a number
  • No special characters: Avoid $, spaces, null characters
  • Max length: 120 characters (including database name)
  • Case sensitive: "Users" and "users" are different
  • Use plural names: "users", "products", "orders" 
// Good collection names
db.users.insertOne({})
db.product_reviews.insertOne({})
db.orderHistory.insertOne({})

// Bad collection names (avoid)
db.123users.insertOne({})      // starts with number
db.user data.insertOne({})     // has space
db.$special.insertOne({})      // starts with $

🔹 Complete Example

Here's a practical example of working with collections: 

// Switch to database
use blogDB

// Create multiple collections
db.createCollection("posts")
db.createCollection("comments")
db.createCollection("users")

// Add data to collections
db.posts.insertOne({
    title: "Getting Started with MongoDB",
    content: "MongoDB is a NoSQL database...",
    author: "Jane Doe"
})

db.users.insertOne({
    username: "janedoe",
    email: "[email protected]"
})

// View all collections
show collections

// Count documents
db.posts.countDocuments()
db.users.countDocuments()

Output:

comments

posts

users

Posts: 1, Users: 1

🧠 Test Your Knowledge

What happens when you insert a document into a non-existent collection?