API with NestJS #43. Introduction to MongoDB

JavaScript MongoDB NestJS

This entry is part 43 of 158 in the API with NestJS

So far, in this series, we’ve focused on working with SQL and the Postgres database. While PostgreSQL is an excellent choice, it is worth checking out the alternatives. In this article, we learn about how MongoDB works and how it differs from SQL databases. We also create a simple application with MongoDB and NestJS.

You can find the source code from the below article in this repository.

MongoDB vs. SQL Databases

The design principles of MongoDB differ quite a bit from traditional SQL databases. Instead of representing data with tables and rows, MongoDB stores it as JSON-like documents. Therefore, it is relatively easy to grasp for developers familiar with JavaScript.

Documents in MongoDB consist of key and value pairs. The significant aspect of them is that the keys can differ across documents in a given collection. This is a big difference between MongoDB and SQL databases. It makes MongoDB a lot more flexible and less structured. Therefore, it can be perceived either as an advantage or a drawback.

Advantages and drawbacks of MongoDB

Since MongoDB and SQL databases differ so much, choosing the right tool for a given job is crucial. Since NoSQL databases put fewer restrictions on the data, it might be a good choice for an application evolving quickly. We still might need to update our data as our schema changes.

For example, we might want to add a new property containing the user’s avatar URL. When it happens, we still should deal with documents not containing our new property. We can do that by writing a script that puts a default value for old documents. Alternatively, we can assume that this field can be missing and handle it differently on the application level.

On the contrary, adding a new property to an existing SQL database requires writing a migration that explicitly handles the new property. This might seem like a bit of a chore in a lot of cases. However, with MongoDB, it is not required. This might make the work easier and faster, but we need to watch out and not lose the integrity of our data.

If you want to know more about SQL migrations, check out The basics of migrations using TypeORM and Postgres

SQL databases such as Postgres keep the data in tables consisting of columns and rows. A big part of the design process is defining relationships between the above tables. For example, a user can be an author of an article. On the other hand, MongoDB is a non-relational database. Therefore, while we can mimic SQL-style relationships with MongoDB, they will not be as efficient and foolproof.

Using MongoDB with NestJS

So far, in this series, we’ve used Docker to set up the architecture for our project. We can easily achieve that with MongoDB also.


Above, you can see that we refer to a few variables. Let’s put them into our file:


In the previous parts of this series, we’ve used TypeORM to connect to our PostgreSQL database and manage our data. For MongoDB, the most popular library is Mongoose.

Let’s use Mongoose to connect to our database. To do that, we need to define a URI connection string:


Saving and retrieving data

With MongoDB, we operate on documents grouped into collections. To start saving and retrieving data with MongoDB and Mongoose, we first need to define a schema. This might seem surprising at first because MongoDB is considered schemaless. Even though MongoDB is flexible, Mongoose uses schemas to operate on collections and define their shape.

Defining a schema

Every schema maps to a single MongoDB collection. It also defines the shape of the documents within it.


With the decorator, we can mark our class as a schema definition and map it to a MongoDB collection. We use the decorator to definite a property of our document. Thanks to TypeScript metadata, the schema types for our properties are automatically inferred.

We will expand on the topic of defining a schema in the upcoming articles.

Working with a model

Mongoose wraps our schemas into models. We can use them to create and read documents. For our service to use the model, we need to add it to our module.


We also need to inject the model into our service:


Once we do that, we can start interacting with our collection.

Fetching all entities

The most basic thing we can do is to fetch the list of all of the documents. To do that, we need the method:


Fetching a single entity

Every document we create is assigned with a string id. If we want to fetch a single document, we can use the method:


Creating entities

In the fourth part of this series, we’ve tackled data validation. Let’s create a Data Transfer Object for our entity.


We can now use it when creating a new instance of our model and saving it.


Updating entities

We might also need to update an entity we’ve already created. To do that, we can use the method:


Above, a few important things are happening. Thanks to using the parameter, the method returns an updated version of our entity.

By using , we indicate that we want to replace a whole document instead of performing a partial update. This is what differentiates the PUT and PATCH HTTP methods.

If you want to know more, check out TypeScript Express tutorial #15. Using PUT vs PATCH in MongoDB with Mongoose.

Deleting entities

To delete an existing entity, we need to use the method:


Defining a controller

Once we’ve got our service up and running, we can use it with our controller:


The crucial part above is that we’ve defined the class. With it, we can validate if the provided string is a valid MongoDB id:



In this article, we’ve learned the very basics of how to use MongoDB with NestJS. To do that, we’ve created a local MongoDB database using Docker Compose and connected it with NestJS and Mongoose. To better grasp MongoDB, we’ve also compared it to SQL databases such as Postgres. There are still a lot of things to cover when it comes to MongoDB, so stay tuned!

Series Navigation<< API with NestJS #42. Authenticating users with GoogleAPI with NestJS #44. Implementing relationships with MongoDB >>
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Ramon Hoyo
Ramon Hoyo
1 year ago

Hi! thanks your efforts to create these very usefull documents!

I wondering in the best way to handling mongo versioning with nest, for instance, the __v property of the schemas.

I mean, I want to apply some upgrade logic for some document and update its version.

I don’t know if you have some code example or overview for handling this.

Thanks you so much.