Sequelize Many to Many表，带有3个主键

Question

我在两个表之间有多对多的注册关系：学生和课程是这样的：

var Enrollment = sequelize.define('Enrollment', {
  EnrollmentType: {
    //some attributes
},
{
  associate: function(models){
    models.Person.belongsToMany(models.Course, { as: 'CourseEnroll', through: { model: Enrollment }, foreignKey: 'StudentEnrollId'});
    models.Course.belongsToMany(models.Person, { as: 'StudentEnroll', through: {model: Enrollment},foreignKey: 'CourseEnrollId'});
  }
}

但是我需要在Period Table中添加第三个Key，以便在Enrollment表上有3个唯一的键组合。我试过了：

Enrollment.belongsTo(models.Period, { as: 'PeriodEnroll', foreignKey: 'PeriodEnrollId', primaryKey: true,});

但是没有工作，我也尝试用其他一些表做第三个belongsToMany但是没有用。我该如何存档？