added validation of role
added methods in course model to return teachers and students
changed student_assignment model to say that it belongs to :student, class_name: :user
made course memberships dependent on the user - if the user is removed from the database, their associated course membership goes with it.
working on scopes
added annotate gem and also testing out class methods and removing student_assignments table
tests not passing because no foreign keys in database
trying out a self join table instead of assignments and student_assignments
fixed merge conflicts
fixed merge conflicts