Nextjs and Sequelize database connection singleton problem

I'm trying to use nextjs and sequelize together. It works but I have a problem with each request from the database it opens a connection for DB. I have a Db class and export an instance from this class then use it wherever I need.

import { FindOptions, Model, ModelCtor, Sequelize } from 'sequelize'
import { initModels, Models } from './models';

class Db {
    connection: Sequelize | null = null;
    constructor() {
        this.connection = this.connectDb() 
        initModels(this.connection)
    }

    retrieveAll(model: ModelCtor<Model<any, any>>, options: FindOptions<any> = {}) {
        return model?.findAll(options);
    }

    retrieve(model: ModelCtor<Model<any, any>>, options: FindOptions<any> = {}) {
        return model?.findOne(options);
    }

    connectDb() {
        const dbName = process.env.DB_NAME
        const dbHost = process.env.DB_HOST
        const dbUser = process.env.DB_USER as string       
        const dbPassword = process.env.DB_PASSWORD
        
        const sequelize = new Sequelize({
            dialect: 'postgres',
            host: dbHost,
            port: 5432,
            database: dbName,
            username: dbUser,
            password: dbPassword,
            pool: {
                max: 5,
                idle: 86400000,
            }
        });
        
        try {
            sequelize.authenticate();
            console.log('Connection has been established successfully.');
            return sequelize;
        } catch (error) {
            console.error('Unable to connect to the database:', error);
            return null;
        }
        
    }

    closeConnectionDb() {
        this.connection?.close()
    }
}

export const db = new Db();

For example, I make a query database for login then on homepage I get merchant list from the database. I see two times "Connection has been established successfully.". How can I solve this problem? By the way, it works very well in the production server. I think it's about module caching but I'm not sure.