var UserSchema = new mongoose.Schema({
    firstName: String,
    lastName: String,
    image: {
        type: String,
        default: "http://via.placeholder.com/250x200"
    },
    email: {
        type: String,
        required: [true, "Email missing."],
        unique: true
    },
    kind: {
        type: String,
        enum: ["Admin", "Teacher", "Student"],
        default: "Student"
    },
    isAdmin: {
        type: Boolean,
        default: false
    },
    isPromote: {
        type: Boolean,
        default: false
    },
    username: {
        type: String,
        required: [true, "Username missing."],
        unique: true
    },
    password: String
}, options);

UserSchema.plugin(passportLocalMongoose);

module.exports = mongoose.model("User", UserSchema);

var options = {discriminatorKey: 'kind'};
var StudentSchema = new mongoose.Schema({
    indexNumber: String,
}, options);

module.exports = User.discriminator("Student", StudentSchema);

var CourseSchema = new mongoose.Schema({
    teachers: [{
        type: mongoose.Schema.Types.ObjectId,
        ref: "Teacher"
    }],
    students: [{
        id: {
            type: mongoose.Schema.Types.ObjectId,
            ref: "Student"
        },
        entryDate: {
            type: Date,
            default: Date.now
        }
    }]

Course.findById(req.params.id)
.populate("students.id")
.populate("teachers")
.exec( function(err, foundCourse) {...}

students: [ { _id: 59dbd1259df328279cd5d588, entryDate: 2017-10-12T22:29:48.297Z }, { _id: 59dc07088c6fd636188d87c5, entryDate: 2017-10-12T22:29:48.297Z } ], teachers: [ { _id: 59dbc34832b0041b44a7f648, isPromote: false, isAdmin: false, kind: 'Teacher', image: 'http://via.placeholder.com/250x200' } ]