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Mongoose (ODM) Basics

While MongoDB’s flexibility is powerful, it can also lead to problems. Without any structure, you might:
  • Insert documents with typos in field names (usernmae vs username)
  • Store different types in the same field (sometimes a string, sometimes a number)
  • Miss required fields entirely
  • Have no validation before data reaches the database
Mongoose solves these problems by adding a schema layer on top of MongoDB.

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What is an ODM?

ODM stands for Object Data Modeling—it’s like an ORM (Object Relational Mapping) but for document databases. Mongoose maps JavaScript objects to MongoDB documents and provides:
FeatureBenefit
SchemasDefine structure for your documents
ValidationEnsure data meets requirements before saving
Type castingAutomatically convert types (string “42” → number 42)
MiddlewareRun code before/after save, update, delete
Query helpersClean, chainable API for queries
VirtualsComputed properties not stored in database

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Why Use Mongoose?

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Without Mongoose

// Direct MongoDB driver - no validation
await db.collection('users').insertOne({
  nmae: 'Alice',  // Typo - no warning!
  age: 'twenty-five'  // Wrong type - no warning!
});

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With Mongoose

// Mongoose catches errors before they reach the database
const user = new User({
  nmae: 'Alice',  // Error: 'nmae' is not in schema
  age: 'twenty-five'  // Error: Cast to Number failed
});
await user.save();  // Throws ValidationError
Mongoose adds structure without removing flexibility. You can still use MongoDB’s schemaless features when needed with the Mixed type or strict mode disabled.

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Installation

npm install mongoose

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Connecting

const mongoose = require('mongoose');

mongoose.connect('mongodb://localhost:27017/myapp');

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Defining a Schema

const Schema = mongoose.Schema;

const BlogPostSchema = new Schema({
  title: String,
  author: String,
  body: String,
  date: { type: Date, default: Date.now },
  hidden: Boolean,
  meta: {
    votes: Number,
    favs: Number
  }
});

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Creating a Model

const BlogPost = mongoose.model('BlogPost', BlogPostSchema);

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Creating a Document

const post = new BlogPost({
  title: 'Hello World',
  author: 'Alice',
  body: 'This is my first post.'
});

await post.save();

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Querying

// Find all
const posts = await BlogPost.find();

// Find where author is Alice
const alicePosts = await BlogPost.find({ author: 'Alice' });

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Validation

Mongoose has built-in validation. 
const UserSchema = new Schema({
  username: {
    type: String,
    required: true,
    minlength: 5
  },
  age: {
    type: Number,
    min: 18,
    max: 65
  }
});

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Summary

  • Mongoose provides structure to MongoDB documents in Node.js.
  • Schemas define the shape of documents.
  • Models provide an interface to the database.
  • Mongoose handles validation, casting, and business logic hooks.