In this assessment, you will be building a tool for helping teachers organize the courses they teach and the students in each. As this is designed from a teacher's perspective, a course has many students, and each student belongs to a specific course (though we know you may actually be taking multiple courses).

Your task is to build a set of collaborating Ruby objects, one to handle instances of Courses and one to handle Students. Write your solution in lib/oo_course_domain.rb following the instructions below.

Create an @@all class variable set to an array.

Initialize student instances with a string name. On initialization, the new student instance should be pushed into the @@all class variable.

Write a .all class method that returns this variable. This variable will be used to contain all student instances that have been created.

Students instances should have name, course and grade accessors. Since we have only assigned name when initializing, course and grade by default should return nil until they are assigned.

Write a has_grade? method that checks if the student's grade is assigned or is still the default nil. Return true if student has a grade, false if not.

Create an @@all class variable set to an array.

Course instances should be initialized with a name argument. On initialization, the new course instance should be pushed into the @@all class variable.

Write a .all class method that returns an array of all course instances that have been created.

Course instances should have a name attribute accessor.

Write an #enroll_student instance method that takes in a student instance as an argument and associates that student with the course. It should do this by telling the student that it belongs to this particular course.

Write an #enroll_student_by_name instance method that takes in a student name as a string, creates a new student instance, and associates that student with the course instance.

Write an #add_grade instance method that takes two arguments, a student instance and an Integer grade, and sets the student's grade.

Write a #students instance method that returns a collection of student instances that are enrolled in a course.

Write an #all_existing_grades instance method that returns a collection of all grades, leaving out any ungraded students.

Write an #all_students_graded? instance method that does one of three things:

• Returns true if all enrolled students have been assigned grades
• Returns false if some enrolled students have yet to receive grades
• Returns false if there are no students enrolled in the course

Write an #average_grade method that does one of two things:

• if grading is finished, returns an average of all student grades. To calculate average, add all values together and divide by the number of values
• if grading is not finished, returns a string 'Grading still in progress.'