MongoDB with Deno

Introduction

MongoDB lets you store and manage data easily. Using it with Deno helps you build fast and simple apps that save and get data.

You want to save user info like names and emails in a web app.

You need to keep track of tasks or notes in a to-do list app.

You want to store product details for an online store.

You want to quickly add or find data without complex setup.

Syntax

Deno

import { MongoClient } from "https://deno.land/x/mongo@v0.31.1/mod.ts";

const client = new MongoClient();
await client.connect("mongodb://localhost:27017");

const db = client.database("myDatabase");
const users = db.collection("users");

// Insert a document
await users.insertOne({ name: "Alice", age: 25 });

// Find a document
const user = await users.findOne({ name: "Alice" });

Use await because MongoDB operations are asynchronous.

Change the connection string to match your MongoDB server address.

Examples

Adds a new user named Bob with age 30 to the users collection.

Deno

await users.insertOne({ name: "Bob", age: 30 });

Finds the first user document where the name is Bob.

Deno

const user = await users.findOne({ name: "Bob" });

Gets all user documents as an array.

Deno

const allUsers = await users.find({}).toArray();

Sample Program

This program connects to MongoDB, adds a user named Charlie, then finds and prints that user.

Deno

import { MongoClient } from "https://deno.land/x/mongo@v0.31.1/mod.ts";

const client = new MongoClient();
await client.connect("mongodb://localhost:27017");

const db = client.database("testDB");
const users = db.collection("users");

// Insert a user
await users.insertOne({ name: "Charlie", age: 28 });

// Find the user
const user = await users.findOne({ name: "Charlie" });

console.log(user);

Important Notes

Make sure MongoDB server is running before connecting.

Use console.log to see results in Deno's terminal.

MongoDB stores data as JSON-like documents, which fits well with JavaScript and Deno.

Summary

MongoDB stores data in collections of documents.

Deno can connect to MongoDB using the MongoClient from the mongo module.

You use insertOne to add data and findOne to get data.

Practice

1. What is the primary way MongoDB stores data when used with Deno?

easy

A. In collections of documents

B. In tables with rows and columns

C. As key-value pairs in a flat file

D. In XML files

Solution

1. Step 1: Understand MongoDB data structure

   MongoDB stores data in collections, which are groups of documents.

2. Step 2: Compare with other storage types

   Unlike SQL databases, MongoDB does not use tables but uses collections of JSON-like documents.

3. Final Answer:

   In collections of documents -> Option A

4. Quick Check:

   MongoDB = collections of documents [OK]

Hint: Remember MongoDB uses collections, not tables [OK]

Common Mistakes:

• Confusing MongoDB with SQL tables
• Thinking MongoDB stores data in flat files
• Assuming XML storage

2. Which of the following is the correct way to create a MongoClient instance in Deno?

easy

A. const client = new mongo.Client();

B. const client = MongoClient();

C. const client = mongoClient();

D. const client = new MongoClient();

Solution

1. Step 1: Check MongoClient instantiation syntax

   In Deno, you create a new MongoClient instance using the 'new' keyword and the exact class name 'MongoClient'.

2. Step 2: Validate option correctness

   const client = new MongoClient(); uses 'new MongoClient()', which is correct. Other options have incorrect casing or missing 'new'.

3. Final Answer:

   const client = new MongoClient(); -> Option D

4. Quick Check:

   Use 'new MongoClient()' to create client [OK]

Hint: Use 'new' keyword with exact class name MongoClient [OK]

Common Mistakes:

• Omitting 'new' keyword
• Wrong casing in 'MongoClient'
• Using incorrect module or variable names

3. Given the following Deno MongoDB code snippet, what will be the output?

const client = new MongoClient();
await client.connect("mongodb://localhost:27017");
const db = client.database("testdb");
const users = db.collection("users");

await users.insertOne({ name: "Alice", age: 30 });
const user = await users.findOne({ name: "Alice" });
console.log(user?.age);

medium

A. undefined

B. null

C. 30

D. Error: user not found

Solution

1. Step 1: Insert a document and then find it

   The code inserts a user document with name 'Alice' and age 30, then queries for that user.

2. Step 2: Access the age field from the found document

   The findOne call returns the document, so user?.age is 30, which is logged.

3. Final Answer:

   30 -> Option C

4. Quick Check:

   Inserted age 30, found age 30 [OK]

Hint: Inserted document fields appear in findOne result [OK]

Common Mistakes:

• Assuming findOne returns undefined if document exists
• Confusing null and undefined
• Forgetting to await async calls

4. Identify the error in this Deno MongoDB code snippet:

const client = new MongoClient();
client.connect("mongodb://localhost:27017");
const db = client.database("testdb");
const users = db.collection("users");
await users.insertOne({ name: "Bob" });

medium

A. Missing await before client.connect

B. Wrong method name 'database' instead of 'db'

C. insertOne should not be awaited

D. MongoClient cannot connect to localhost

Solution

1. Step 1: Check async connection call

   The connect method returns a promise and must be awaited to ensure connection before proceeding.

2. Step 2: Validate other code parts

   Method 'database' is correct, insertOne is async and should be awaited, and MongoClient can connect to localhost.

3. Final Answer:

   Missing await before client.connect -> Option A

4. Quick Check:

   Always await async connect calls [OK]

Hint: Always await async connect calls [OK]

Common Mistakes:

• Forgetting await on async connect
• Confusing method names
• Not awaiting insertOne

5. You want to insert multiple user documents in a MongoDB collection using Deno. Which approach correctly inserts multiple documents efficiently?

hard

A. Use insertOne in a loop for each document

B. Use insertMany with an array of documents

C. Call insertOne once with an array of documents

D. Use findOne to insert documents

Solution

1. Step 1: Understand bulk insert methods

   insertMany is designed to insert multiple documents at once efficiently.

2. Step 2: Compare options

   insertOne inserts a single document; calling it in a loop is inefficient. insertOne cannot take an array. findOne is for querying, not inserting.

3. Final Answer:

   Use insertMany with an array of documents -> Option B

4. Quick Check:

   Bulk insert = insertMany [OK]

Hint: Use insertMany for multiple documents at once [OK]

Common Mistakes:

• Using insertOne repeatedly instead of insertMany
• Passing array to insertOne
• Confusing findOne with insert methods