store.js

import { observable, computed } from 'mobx'

const store = observable({
    students: observable.map(),
    courses: observable.map(),
    enrollment: observable.map(),

    addStudent(student) {
        if (this.students.has(student.id)) {
            throw new Error('student already exists')
        } else {
            this.students.set(student.id, student)
        }
    },
    updateStudent(student) {
        this.students.set(student.id, student)
    },
    removeStudent(id) {
        this.students.delete(id)
    },

    addCourse(course) {
        if (this.courses.has(course.id)) {
            throw new Error('course already exists')
        } else {
            this.courses.set(course.id, course)
        }
    },
    updateCourse(course) {
        this.courses.set(course.id, course)
    },
    removeCourse(id) {
        this.courses.delete(id)
    },

    enrollStudent(courseId, studentId) {
        if (!this.enrollment.get(courseId)) {
            this.enrollment.set(courseId, observable.array([]))
            this.enrollment.get(courseId).push(studentId)
        } else if (!this.enrollment.get(courseId).includes(studentId)){
            this.enrollment.get(courseId).push(studentId)
        }
    },
    unenrollStudent(courseId, studentId) {
        this.enrollment.get(courseId).remove(studentId)
    },
    enrolledStudents(courseId) {
        return computed(() =>
                        this.enrollment.get(courseId).map(n =>
                                                          this.students.get(n)))
            .get()    
     }
})

export default store