Mongoose middleware (pre/post hooks) in Express

Introduction

Mongoose middleware lets you run code before or after certain actions on your data. It helps automate tasks like validation, logging, or modifying data.

You want to check or change data before saving it to the database.

You need to log information every time a document is deleted.

You want to update related data automatically after a document is updated.

You want to run cleanup tasks before removing a document.

You want to validate or modify data before running queries.

Syntax

Express

schema.pre('action', function(next) {
  // code to run before action
  next();
});

schema.post('action', function(doc, next) {
  // code to run after action
  next();
});

pre runs before the action, post runs after.

Always call next() to continue the process.

Examples

Sets a creation date before saving a user.

Express

userSchema.pre('save', function(next) {
  this.createdAt = new Date();
  next();
});

Logs a message after a user is removed.

Express

userSchema.post('remove', function(doc, next) {
  console.log(`User ${doc._id} was removed.`);
  next();
});

Updates the updatedAt field before updating a user.

Express

userSchema.pre('findOneAndUpdate', function(next) {
  this.set({ updatedAt: new Date() });
  next();
});

Sample Program

This example shows three middleware hooks: one sets createdAt before saving, one sets updatedAt before updating, and one logs a message after removing a user.

Express

import mongoose from 'mongoose';

const userSchema = new mongoose.Schema({
  name: String,
  createdAt: Date,
  updatedAt: Date
});

// Pre-save hook to set createdAt
userSchema.pre('save', function(next) {
  if (!this.createdAt) {
    this.createdAt = new Date();
  }
  next();
});

// Pre-update hook to set updatedAt
userSchema.pre('findOneAndUpdate', function(next) {
  this.set({ updatedAt: new Date() });
  next();
});

// Post-remove hook to log removal
userSchema.post('remove', function(doc) {
  console.log(`User ${doc._id} removed.`);
});

const User = mongoose.model('User', userSchema);

async function run() {
  await mongoose.connect('mongodb://localhost:27017/testdb');

  // Create and save user
  const user = new User({ name: 'Alice' });
  await user.save();

  // Update user
  await User.findOneAndUpdate({ _id: user._id }, { name: 'Alice Updated' });

  // Remove user
  await user.remove();

  await mongoose.disconnect();
}

run().catch(console.error);

Important Notes

Middleware functions must call next() to continue the operation.

Use pre hooks to modify data before actions, and post hooks for side effects like logging.

For query middleware like findOneAndUpdate, use this.set() to change update data.

Summary

Mongoose middleware runs code before or after database actions.

Use pre hooks to prepare or validate data.

Use post hooks to perform tasks after actions, like logging.