An Introduction to Sequelize, a Node.js Object-Relational Mapper (ORM) | by Todd Demone | Apr, 2022

A greater approach to handle knowledge from inside your net software

Sequelize is a well-liked Object-Relational Mapper (ORM) for Node.js. An ORM is a library answerable for managing knowledge in purposes, particularly purposes that (a) are written in an object-oriented programming language and (b) persistently retailer knowledge in a database.

This text begins with a primer on ORMs, adopted by an outline of Sequelize’s fundamental performance, together with:

  • set up of Sequelize;
  • connecting to a database;
  • defining fashions;
  • creating mannequin cases;
  • querying fashions;
  • creating associations between fashions; and
  • queries involving associations.

This text assumes some familiarity with:

  • object-oriented programming (OOP);
  • JavaScript; and
  • Structured Question Language (SQL).

The next definition, though written in respect of a particular ORM, may be utilized to ORMs as a complete:

The SQLAlchemy Object Relational Mapper presents a way of associating user-defined Python courses with database tables and cases of these courses (objects) with rows of their corresponding tables. It features a system that transparently synchronizes all adjustments in state between objects and their associated rows, known as a unit of labor, in addition to a system for expressing database queries by way of the user-defined courses and their outlined relationships between one another.

An ORM creates a mapping between two fairly completely different representations of knowledge: the courses and sophistication cases (objects) of an OOP language, and the tables and rows of a relational database.

ORMs hold the 2 teams synchronized, so every time a category or occasion adjustments in your software, the corresponding database desk or row is modified as effectively.

Database statements and queries may be written within the programming language of your alternative, and the ORM converts these directions into SQL code that’s understood by the database.

Writing advanced SQL queries could be a lengthy and cumbersome course of, so utilizing an ORM can save growth time. The efficacy of utilizing ORMs vs. merely writing uncooked SQL is mentioned here, here, and here.

Many libraries implement the ORM approach. Within the Node.js world, there’s Prisma, Bookshelf, Waterline, and Objection, amongst others. Sequelize is the most well-liked Node.js ORM for SQL databases (26k stars on GitHub on the time of writing).

Sequelize’s GitHub page consists of the next description of the library:

Sequelize is an easy-to-use and promise-based Node.js ORM device for Postgres, MySQL, MariaDB, SQLite, DB2, Microsoft SQL Server, Snowflake, and IBM i. It options strong transaction assist, relations, keen and lazy loading, learn replication, and extra.

The examples on this article use the SQLite3 database.

Sequelize may be added to a Node.js undertaking utilizing Node Package deal Supervisor (npm). A driver for a selected database additionally must be put in. Typing the next instructions right into a terminal will create a brand new Node.js undertaking with Sequelize put in:

We have to create an occasion of the Sequelize class to connect with our database. The Sequelize constructor takes completely different parameters relying on what sort of database we’re utilizing. When utilizing the SQLite3 database, we cross two parameters — the database dialect (‘sqlite’), and the trail to our SQLite3 storage file:

Utilizing a full-featured Relational Database Administration System (RDBMS) resembling MySQL and PostgreSQL requires us to cross extra parameters than in our SQLite3 instance:

A mannequin is a element of some software program design patterns. Fashions are answerable for representing and managing the information in an software. In Sequelize, every mannequin is a JavaScript class and represents a desk in a database. ORMs like Sequelize mechanically create a database desk every time a mannequin is outlined.

Defining a Mannequin

To outline a mannequin in Sequelize, we first create a category that extends Sequelize’s Mannequin class, then we name the init technique on that class, passing in our mannequin attributes and different choices:

Alternatively, fashions may be outlined by calling sequelize.outline(modelName, attributes, choices).

Mannequin attributes

Including an attribute to a mannequin creates a corresponding column within the database desk. Within the above instance, identify, age, and favoriteColor are outlined as Person mannequin attributes and subsequently are additionally column names in our customers desk.

Mannequin knowledge sorts

Every of a mannequin’s attributes should have an information sort. Importing DataTypes right into a JavaScript file offers entry to Sequelize’s built-in knowledge sorts, together with (amongst others) String, Textual content, Boolean, Integer, Decimal, Float, Date.

Default values

Setting the defaultValue choice on an attribute ensures that, when creating an occasion of a mannequin, the attribute is mechanically assigned the default worth if no different worth is inputted.

Computerized desk naming

Once we outlined a Person mannequin in our instance above, a desk named ‘Customers’ was mechanically added to the database.

Though our code by no means explicitly gave our desk a reputation, underneath the hood, Sequelize creates a desk identify that’s the plural of the mannequin identify.

Computerized creation of ‘id’ (main key)

With Sequelize we don’t must specify an id (main key) attribute when defining a mannequin — Sequelize mechanically consists of an id column in our desk.

Computerized creation of ‘createdAt’ and ‘updatedAt’

Once we outline a mannequin, Sequelize mechanically consists of and manages two columns of sort DATE in our desk — createdAt and updatedAt.


Sequelize offers predefined validators that may be set when defining a mannequin attribute. We are able to additionally write customized validators. Validators examine inputted knowledge earlier than an SQL operation is executed. If the information doesn’t cross the validation check, the SQL operation isn’t executed.

As an example, if, when defining a mannequin, the allowNull: false validator is ready on a column, that column can’t be left clean when creating an occasion.

One other widespread validation is distinctive: true. As an example, if that choice had been set on a username column, an try to insert an present username would throw an error.

As soon as we’ve got outlined a mannequin, we will create cases based mostly on that mannequin. Do not forget that a mannequin is a JavaScript class, and the mannequin cases are cases (objects) of that class. These cases map to rows (data) in our database desk.

When reviewing Sequelize’s occasion strategies under, you’ll discover that lots of them are asynchronous. The strategies return a Promise object, and Promise strategies resembling then and catch may be chained to the tip of the asynchronous technique to take care of its return worth.

Creating mannequin cases (and INSERT-ing them into the database)

We are able to create an occasion of an outlined mannequin utilizing the construct technique.

Though the construct technique creates an occasion of a mannequin, it doesn’t create a file within the database desk. The construct technique have to be adopted by a name to the save technique to save lots of the occasion to the database.

A shortcut does exist, nonetheless — the create technique does the work of each construct and save in a single technique:

Updating cases and database data

A Sequelize occasion may be up to date in the identical method that an object may be up to date in JavaScript.

The identify attribute change to ‘Ada’ have to be adopted by a name to the save technique to save lots of the change to the database.

Like create, the replace technique is a shortcut for concurrently updating a file and saving the change to the database:

Deleting cases and database data

The destroy technique deletes an occasion and removes it from the database:

Sequelize offers many finder strategies, every of which generates a SQL SELECT question. As an example, utilizing the findAll technique generates a SELECT * FROM ... sort question in SQL:

findByPk is one other finder technique. It returns a single desk file based mostly on the id (main key) handed in as a parameter:

Every finder technique returns a Promise containing occasion(s) of the mannequin that the strategy was known as on. In different phrases, you don’t simply obtain the database outcomes on a question — Sequelize wraps every outcome from the database question in an occasion object. Every returned occasion has a number of comfort strategies out there to it.

Finder strategies like findByPk return a single mannequin occasion, whereas findAll returns an array of mannequin cases.

Filtering queries with ‘the place’

Sequelize’s question strategies may be filtered utilizing the the place choice.

Notice that it wasn’t obligatory to incorporate an equality operator in our instance code — Sequelize assumes equality if no operator is offered.

However when we have to embrace an operator with the the place choice, can we merely use JavaScript built-in operators? No. We have to use Sequelize’s outlined operators. Sequelize’s Op (which have to be imported into your file) offers many operators which are equal to SQL’s WHEREassociated operators. Listed below are only a few:

Right here is an instance of a question that makes use of the the place choice in addition to Sequelize’s Op.and operator:

Relational databases are beneficial as a result of they’ll create associations (relations) between tables. The kinds of associations that may be created embrace One-to-Many, One-to-One, and Many-to-Many.

Sequelize offers features that, when utilized in mixture, can replicate the usual database associations between two fashions.

The 4 affiliation features are as follows:

  • The hasOne affiliation
  • The belongsTo affiliation
  • The hasMany affiliation
  • The belongsToMany affiliation


The hasOne and belongsTo features can be utilized together to create a One-to-One relationship between two tables. If no international key choice is handed, Sequelize mechanically assigns a international key to the mannequin that calls belongsTo. On this instance, the Profile mannequin will get a international key of UserId:


The hasMany and belongsTo features can be utilized together to create a One-to-Many relationship between two tables. Once more, the mannequin that calls belongsTo will get the international key — on this case, the Remark mannequin will get a international key of UserId.


For Many-to-Many relationships, Sequelize makes use of the idea of a Junction Mannequin a.okay.a. a be part of desk. On this state of affairs, Sequelize creates a 3rd, new mannequin (and consequently a 3rd desk within the database) containing two international key columns, one for every of our linked tables.

Film and Actor fashions present an amazing instance of a many-to-many relationship since an actor can have many motion pictures and a film can have many actors. On this instance, we use the belongsToMany perform to create a Junction Mannequin that retains tracks of the associations between the 2 fashions. We cross belongsToMany a reputation for our Junction Mannequin ( ‘ActorMovies’). The mannequin will comprise the international keys ‘movieId’ and ‘actorId’.

What if we’re working with a mannequin and we need to fetch knowledge from a second mannequin that’s related to (associated to) the primary mannequin? Or what if we need to fetch knowledge from two related fashions abruptly? In Sequelize, we will accomplish these targets by utilizing one among two methods: lazy loading and keen loading.

To reveal each methods, we are going to create an instance involving a One-to-Many relationship between two fashions: Person and Remark (a consumer has many feedback):

Lazy loading

Lazy loading includes fetching Person knowledge first, then fetching Remark knowledge later, solely once we want it.

Right here is the console.log() output:

The feedback variable accommodates an array of objects, every object representing a single remark belonging to the consumer with an id of 1.

The getComments() technique is created by Sequelize to let you seize knowledge that’s associated to the mannequin you might be working with. The identify of the strategy is generated from the identify of the related mannequin. Since our related mannequin is known as ‘Remark’, our technique is mechanically named ‘getComments()’.

Keen loading

Keen loading includes fetching knowledge from a number of tables which have relations with one another.

If we wished to do that utilizing SQL, we must assemble a question with a number of JOIN clauses.

In Sequelize, we will use a finder technique resembling findAll or findByPk and incorporate the embrace choice into that technique to fetch knowledge from each fashions.

Once we do that, Sequelize returns objects which have fields containing knowledge from each of the related fashions.

For our instance, we are going to fetch all feedback together with every remark’s related consumer:

The console.log() output under reveals that Sequelize has returned an object containing the Remark fields physique, id and userId plus an extra discipline known as consumer which accommodates the Person occasion and all of its attributes:

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