https://jobinandjismi.atlassian.net/browse/GED-542
Sequelize eager loading is a way to fetch data from multiple tables that have relations between each other.
When you use the eager loading technique, the generated SQL query will have one or more JOIN clauses.
Suppose you have two tables in your database named Users and Invoices as follows
# Users
| id | firstName |
| 1 | Nathan |
| 2 | John |
#Invoices
| id | amount | userId |
| 1 | 300 | 1 |
| 2 | 100 | 2 |
The Invoices table stores the UserId to let viewers know which user is creating the invoice row.
This means each Users row may have one or more Invoices rows.
To fetch data from both tables with one query, you need to define the relationship (or association) between the two tables in Sequelize models.
First, create the model for both tables as shown below:
const Invoice = sequelize.define(
“Invoice”,
{ amount: Sequelize.INTEGER },
{ timestamps: false }
);
const User = sequelize.define(
“User”,
{ firstName: Sequelize.STRING },
{ timestamps: false }
);
Once both models are created, define the correct association between the two models.
In this case, User.hasMany() and Invoice.belongsTo() must be defined so that you can fetch both tables from whichever model you choose to query the data:
User.hasMany(Invoice);
Invoice.belongsTo(User);
Now you just need to query the data from the models.
You need to specify the associated model to the main model using the include option as follows:
const user = await User.findByPk(1, { include: Invoice });
console.log(user.toJSON());
The SQL query generated by the findByPk() method above will be like this:
You can also query the User model data from the Invoice model as follows:
const inv = await Invoice.findOne({ where: { amount: 300 }, include: User });
console.log(inv.toJSON());
The output of the findOne() method call above will be like this:
{
id: 1,
amount: 300,
UserId: 1,
User: {
id: 1,
firstName: ‘Nathan’
}
}
And that’s how the eager loading works in Sequelize. It allows you to generate and execute an SQL script that retrieves data from multiple tables at once.
const User = sequelize.define(
“User”,
{ firstName: Sequelize.STRING },
{ timestamps: false },
);
const Invoice = sequelize.define(
“Invoice”,
{ amount: Sequelize.INTEGER },
{ timestamps: false },
);
User.hasMany(Invoice);
Invoice.belongsTo(User);
const user = await User.findByPk( 1, { include: Invoice });
console.log(user.toJSON());
const inv = await Invoice.findOne( { where: { amount: 300 }, include: User });
console.log(inv.toJSON());
const user = await User.findByPk(1, { include: [Role, Invoice, /* … */] });