Overview
Sequelize is a powerful Object-Relational Mapping (ORM) library for Node.js. It allows developers to work with relational databases like MySQL, PostgreSQL, and SQLite in an object-oriented fashion. One common requirement is to retrieve data from linked tables as a flat object, which can be simpler to work with than nested representations. This tutorial will guide you through the process of flattening associated data using Sequelize.
Basic Flat Query
To start, we will fetch data from a primary table with a simple association. Consider a model User which has a one-to-one relationship with a Profile model. Using Sequelize’s include option, we can easily fetch the associated data. However, by default, Sequelize nests the result:
User.findOne({
include: [
{
model: Profile,
as: 'profile',
},
],
}).then(user => {
console.log(user);
});
To flatten this object, we can use Sequelize’s attributes functionality to specify the fields we want and concatenate them with the fields from the main User model:
User.findOne({
attributes: [
'id', 'username', [Sequelize.col('profile.bio'), 'bio'],
],
include: [
{
model: Profile,
attributes: [],
},
],
}).then(user => {
console.log(user.get({ plain: true }));
});
Advanced Flat Query Techniques
When dealing with more complex associations, such as a many-to-many relationship, we may want to include specific attributes from the join table or apply aggregations. Here’s an advanced example:
User.findAll({
attributes: {
include: [
[Sequelize.literal('(`roles->UserRole`.role_id)`'), 'roleId'],
],
},
include: [
{
model: Role,
through: { attributes: [] },
attributes: [],
},
],
}).then(users => {
users.forEach(user => {
console.log(user.get({ plain: true }));
});
});
In this example, we have purposefully excluded attributes from the associated Role model and the join table UserRole, but we have manually included the role_id from the join table in our main result set.
Conclusion
Flattening object results with associations in Sequelize requires a good understanding of the library’s querying capabilities. This tutorial has shown you how to get started with basic flattening and move on to more advanced techniques. Always test your queries thoroughly to ensure they meet your application’s requirements and performance goals.
