In Sequelize, the data of associated models can essentially be fetched in two ways, through eager loading and lazy loading.

Eager loading refers to querying all the data of a model and its associated model at once, which in SQL translates to a LEFT OUTER JOIN query.

Lazy loading, on the other hand, refers to querying the data of a model, and later on, querying the data of its associated model only when you really need it.

Eager loading

In Sequelize, eager loading is done by querying a model using query methods, such as findOne() and findAll(), and specifying the associated model through the include property of the configuration object:

await ModelA.findOne|findAll({  include: {    model: ModelB  }});

Where:

• ModelA is a model instance associated with another model through the hasOne() or hasMany() method.

• ModelB is a model instance associated with another model through the belongsTo() method.