Introduction

In this tutorial, we will learn how to query data from MongoDB using the find() and findOne() methods. By the end of this lesson, you will be able to retrieve and filter data from your MongoDB database based on different conditions.

What you will learn
- How to use find() method to retrieve multiple documents from a collection.
- How to use findOne() method to retrieve a single document from a collection.
- How to filter data using these methods.

Prerequisites
- Basic knowledge of JavaScript.
- Understanding of MongoDB and its basic operations.
- MongoDB and Node.js installed on your local machine.

Step-by-Step Guide

Concepts
- find(): This method is used in MongoDB to retrieve multiple documents from a collection. It returns all documents that match the query criteria. If no criteria are specified, it returns all documents.
- findOne(): This method, as the name suggests, returns only one document that matches the query criteria. If multiple documents match the criteria, it returns the first occurrence.
- Query Filters: With both find() and findOne(), you can provide a query object to filter the results.

Code Examples

Example 1: Using find() without any filter

// Import MongoDB module
const MongoClient = require('mongodb').MongoClient;

// URL of the MongoDB database
const url = "mongodb://localhost:27017/";

// Connect to the MongoDB server
MongoClient.connect(url, function(err, db) {
  if (err) throw err;
  var dbo = db.db("mydb");

  // Find all documents in the "customers" collection
  dbo.collection("customers").find({}).toArray(function(err, result) {
    if (err) throw err;
    console.log(result);
    db.close();
  });
});

This script connects to the MongoDB server, selects the "mydb" database, and retrieves all documents from the "customers" collection.

Example 2: Using findOne() without any filter

// Connect to the MongoDB server
MongoClient.connect(url, function(err, db) {
  if (err) throw err;
  var dbo = db.db("mydb");

  // Find the first document in the "customers" collection
  dbo.collection("customers").findOne({}, function(err, result) {
    if (err) throw err;
    console.log(result);
    db.close();
  });
});

This script does the same as the previous one, but it only retrieves the first document from the "customers" collection.

Example 3: Using find() with a filter

// Connect to the MongoDB server
MongoClient.connect(url, function(err, db) {
  if (err) throw err;
  var dbo = db.db("mydb");
  var query = { address: "Park Lane 38" };

  // Find all documents in the "customers" collection that have the address "Park Lane 38"
  dbo.collection("customers").find(query).toArray(function(err, result) {
    if (err) throw err;
    console.log(result);
    db.close();
  });
});

This script retrieves all documents from the "customers" collection that have the address "Park Lane 38".

Summary

In this tutorial, we have learned how to use find() and findOne() methods to retrieve data from MongoDB. We also learned how to filter the results using a query object.

Practice Exercises

1. Exercise: Write a script to retrieve all documents from the "products" collection that have a price greater than 50.

Solution:

var query = { price: { $gt: 50 } };
dbo.collection("products").find(query).toArray(function(err, result) {
  if (err) throw err;
  console.log(result);
  db.close();
});

1. Exercise: Write a script to retrieve the first document from the "orders" collection that was ordered by "John Doe".

Solution:

var query = { orderedBy: "John Doe" };
dbo.collection("orders").findOne(query, function(err, result) {
  if (err) throw err;
  console.log(result);
  db.close();
});

Keep practicing with different collections and query conditions. Remember that MongoDB also supports advanced queries with operators like $and, $or, $not, etc. Try to use them in your exercises.