sequelize-middleware/models/cars.js

module.exports = (sequelize, DataTypes) => {
  const Cars = sequelize.define(
    "Cars",
    {
      name: DataTypes.STRING,
      year: DataTypes.INTEGER,
      active: DataTypes.BOOLEAN,
      brandId: {
        type: DataTypes.INTEGER,
        references: {
          model: "Brands",
          key: "id"
        }
      }
    },
    {}
  );

  Cars.associate = function(models) {
    Cars.belongsTo(models.Brands, {
      as: "Brand",
      foreignKey: "brandId"
    });
    Cars.belongsToMany(models.Colors, { through: models.CarsColors });
  };

  return Cars;
};