relationshipTester.js

const models = require('./models');
const User = models.User;
const Company = models.Company;
const WorkingDay = models.WorkingDay;

let currentDate = new Date();

WorkingDay.bulkCreate([
  {
    weekDay: 'Monday',
    workingDate: currentDate,
    isWorking: true
  },
  {
    weekDay: 'Tuesday',
    workingDate: currentDate,
    isWorking: true
  },
  {
    weekDay: 'Wednesday',
    workingDate: currentDate,
    isWorking: false
  }
])
.then((workingDays) => {
  User.findAll({where: {id: [1, 2, 3]}, include: ['days']})
  .then((users) => {
    // For user 1, 2 and 3 set the sames workingDays
    users.forEach(user => {
      user.setDays(workingDays) // workingDays is an array (one user hasMany workingDays)
      .then((joinedUsersWorkingDays) => {
        console.log(joinedUsersWorkingDays)
      })
      .catch((err) => console.log("Error while joining Users and WorkingDays : ", err))
    });
  })
  .catch((err) => console.log("Error while Users search : ", err))
})
.catch((err) => console.log("Error while WorkingDay creation : ", err))