const User = this.sequelize.define('User', {
    id: {
      type: DataTypes.INTEGER(11),
      allowNull: false,
      primaryKey: true,
      autoIncrement: true,
    },
    username: {
        type: Sequelize.STRING,
        unique: true,
       field: 'the_user_name_field'
    }
});
const Task = this.sequelize.define('Task', {
    title: Sequelize.STRING,
    username: Sequelize.STRING
});

User.hasOne(Task, { as: 'Task', foreignKey: 'username', sourceKey: 'username' });
User.findOne({ 
    where: { username: 'bob' },
    include: [ 'Task' ],
}); // for mysql still generate sql "ON `user`.`id` = `Task`.`username`"