How to trigger a function whenever a mongoose document is updated

In Mongoose, we can use middleware, also known as pre and post hooks, to trigger callback functions during data updates. These hooks can execute custom logic before (pre) or after (post) certain operations. This is very useful, for example, to validate or modify data before saving a document, or to log after an update operation.

For example, suppose we have a user model, and we want to log the update time after each user data update; we can achieve this using Mongoose's pre hooks.

Here is an example of how to automatically set the updatedAt field before a data update operation using pre hooks:

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

const userSchema = new Schema({
  username: String,
  email: String,
  updatedAt: Date
});

// Use pre hook to set updatedAt field before update operation
userSchema.pre('updateOne', function(next) {
  this.set({ updatedAt: new Date() });
  next();
});

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

// Update operation
User.updateOne({ username: 'johndoe' }, { email: 'johndoe@example.com' })
  .then(result => console.log('Update successful'))
  .catch(err => console.error('Error updating user', err));

In this example, whenever the updateOne method is used to update a user, the pre hook automatically sets the updatedAt field to the current date and time. This ensures that even if we don't explicitly set the updatedAt field in the update call, it is automatically updated.

Additionally, Mongoose supports various other types of middleware, such as save, remove, find, aggregate, etc., which can insert custom logic during the execution of these operations.

This approach makes our database operations more flexible and powerful, helping to maintain data integrity and execute additional logic, such as security checks and data validation.