Intro to Mongoose Middleware

Learning objective: By the end of this lesson, students will be able to implement and utilize middleware in Mongoose for custom behavior during various stages of a document’s lifecycle.

Middleware in Mongoose are functions executed at specific stages of a document’s lifecycle. They allow us to execute code before or after database operations (such as saving a document). This enables custom behavior like data validation, modification, or logging.

Types of middleware

Mongoose offers several types of middleware, each based on where and how they are executed:

• Document Middleware: Occurs at the document level. It is ideal for tasks such as password hashing or data validation before a document is saved.

• Model Middleware: Occurs at the model level. It is helpful for tasks like a cascading delete, where you might want to remove all related comments when a blog post is deleted.

• Query Middleware: Occurs during query operations. It is useful for modifying queries or processing results.

pre and post

All these types of middleware revolve around the use of special functions called pre and post:

• pre: Executed before a specified operation. Useful for modifying data before it is saved to the database.

• post: Executed after an operation is completed. Useful for follow-up tasks after an operation has occurred.

You can visit the Mongoose documentation on middleware for more information.

Implementing middleware

Now that you have an overview of middleware in Mongoose, let’s implement an example. We’ll be working in models/todo.js. Our goal is to add a pre save middleware function. This middleware will run right before a new todo document is saved to the database. It will normalize the string value assigned to the text property.

📚 The term normalize refers to the process of systematically transforming text data into a consistent format for the database.

Update models/todo.js with the following:

// models/todo.js

const mongoose = require('mongoose');

const todoSchema = new mongoose.Schema({
  text: String,
  isComplete: Boolean
});

todoSchema.pre('save', function (next) {
  const docToBeSaved = this
  if (docToBeSaved.text) {
    docToBeSaved.text = docToBeSaved.text[0].toUpperCase() + docToBeSaved.text.slice(1);
  }
  next();
});

const Todo = mongoose.model('Todo', todoSchema);

module.exports = Todo;

The above middleware function uses the pre function, and gets called before a todo document is saved. If the new todo has a text property, the first character of that string will be capitalized. After the condition, we call upon the next() function so that the flow of middleware may continue.

this? In Mongoose middleware, particularly in pre-save hooks, this refers to the instance of the document that is about to be saved to the database. In other words, this contains all the data of the document, including any modifications or transformations that have been applied to it before reaching the save operation.

📚 In the context of Mongoose middleware, next() is a function passed into middleware like pre and post. It signals when a middleware function has completed its task, allowing Mongoose to continue to the next piece of middleware or proceed with the database operation.

Running our middleware

Let’s run our new middleware function. To do so, we’ll need to use the createTodo() function in queries.js.

Modify the todoData so that the first character of the text is lowercase:

// queries.js

const createTodo = async () => {
  const todoData = {
    // Update this line:
    text: "learn React",
    isComplete: false,
  };

  const todo = await Todo.create(todoData);
  console.log("New todo:", todo);
};

Make sure you are calling upon createTodo within the runQueries function:

// queries.js

const runQueries = async () => {
  console.log('Queries running.');
  await createTodo();
};

If you haven’t done so already, be sure to remove or comment out any previous methods being called

To test out our new middleware, run the queries.js file with the following command:

node queries.js

Check your terminal for the following output:

New todo: {
  text: 'Learn React',
  isComplete: false,
  _id: new ObjectId('657b25adc8146427465857d7'),
  __v: 0
}