A Mongoose model is the compiled version of the schema definition that maps directly to a single document in a MongoDB collection. An instance of a model is called a document.

Mongoose models are responsible for querying, creating, updating, and removing documents from the MongoDB database.

Creating a model

To create a Mongoose model from a schema definition, you can use the mongoose.model() function:

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

const schema = new Schema({
    title: String,
    author: String,
    price: Number
});

const Book = mongoose.model('Book', schema);

The first parameter is the singular name of the collection you need a model for, and the second parameter is the schema definition. Mongoose will automatically pluralize and lowercase the name of the collection you specified.

For example, for the above Book model, the actual name of the MongoDB collection will be books. To retrieve all books using the Mongo Shell, you would use the db.books.find() command.

Creating documents

A Mongoose document is an instance of a model that represents a record in a MongoDB collection. To create a new document, you can use the following code:

const book = new Book({
    title: 'Mongoose 101',
    author: 'Atta',
    price: 9.99
});

// Save Book in MongoDB
book.save()
    .then(book => {
        console.log(book);
    }).catch(err => {
        console.log(err);
    });

Alternatively, you could also use the Mongoose model's create() function to create and save new documents:

Book.create({
    title: 'Mongoose 101',
    author: 'Atta',
    price: 9.99
}).then(book => console.log(book))
    .catch(err => console.error(err));

Querying documents

To query documents from a MongoDB collection, you can use the find() method:

Book.find({ author: 'Atta' })
    .then(books => console.log(books))
    .catch(err => console.error(err));

The find() method returns all the matching documents in the collection as an array. To find a single document, you can either use the findById() method or the findOne() method:

// Find Single Document By ID
Book.findById('123')
    .then(book => console.log(book))
    .catch(err => console.error(err));


// Find First Matching Document
Book.findOne({ title: 'Mongoose 101' })
    .then(book => console.log(book))
    .catch(err => console.error(err));

Updating documents

Just like querying, there are multiple ways available to update documents using Mongoose. Each Mongoose model has its own update() method for modifying documents in the collection without returning them back to the application.

Here is an example:

Book.update({ author: 'Atta' }, { author: 'Alex' })
    .then(book => console.log(book))
    .catch(err => console.error(err));

The above example code updates all matching documents in the books collection.

If you want to update a single document in the collection and return it back the application, use the findOneAndUpdate() method instead:

Book.findOneAndUpdate(
    { title: 'Mongoose 101' },
    { title: 'Mongoose Basics' },
    { new: true }
).then(book => console.log(book))
    .catch(err => console.error(err));

Deleting documents

You can use the findOneAndRemove() method to delete a document from a collection. It returns the original document that was removed:

Book.findOneAndDelete({ author: 'John' })
    .then(book => {
        console.log(book);
    }).catch(err => {
        console.log(err);
    });

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