What is Mongoose Schema and how to define and use it?

Mongoose Schema is a core concept in Mongoose used to define the structure, data types, validation rules, and default values of MongoDB documents. Schema itself is not a collection in the database, but a blueprint used to create Models.

Basic Schema Definition

javascript
const mongoose = require('mongoose');
const { Schema } = mongoose;

const userSchema = new Schema({
  name: {
    type: String,
    required: true,
    trim: true
  },
  email: {
    type: String,
    required: true,
    unique: true,
    lowercase: true,
    trim: true
  },
  age: {
    type: Number,
    min: 0,
    max: 120
  },
  createdAt: {
    type: Date,
    default: Date.now
  }
});

Main Schema Properties

1. Field Types: String, Number, Date, Buffer, Boolean, Mixed, ObjectId, Array
2. Validators: required, min, max, enum, match, validate
3. Modifiers: lowercase, uppercase, trim, default
4. Indexes: unique, sparse, index
5. Virtual Fields: Computed fields not stored in database
6. Instance Methods: Methods added to document instances
7. Static Methods: Methods added to model classes
8. Middleware: pre and post hooks

Relationship Between Schema and Model

• Schema is the definition, Model is the constructor
• Create Model via mongoose.model('User', userSchema)
• Model instances are Documents, representing actual documents in the database
• One Schema can create multiple Models (not recommended)

Schema Advantages

1. Data Consistency: Enforce consistent document structure
2. Data Validation: Validate data at application layer
3. Type Safety: Provide type checking and conversion
4. Middleware Support: Execute logic before/after operations
5. Extensibility: Add methods and virtual fields