mysql - Sequelize Many to Many table with 3 Primary keys -

i have many many enrollment relationship between 2 tables: students , courses this:

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'});   } } 

but need add third key period table have 3 unique key combination on enrollment table. tried doing:

enrollment.belongsto(models.period, { as: 'periodenroll', foreignkey: 'periodenrollid', primarykey: true,}); 

but didn't work, tried doing third belongstomany of other tables didn't work. how can archive this?