gistfile1.txt

class Student
   has_many :enrollments
   has_many :courses, :through => :enrollments
   has_many :submissions
   has_many :assignments, :through => :submissions
end

class Enrollment
   belongs_to :student
   belongs_to :course
end

class Course
   has_many :enrollments
   has_many :students, :through => :enrollments
   has_many :assignments
   has_many :submissions, :through => :assignments
end

class Assignment
   belongs_to :course
   has_many :submissions
end

class Submission
   belongs_to :assignment
   belongs_to :student

   named_scope :graded, :conditions => ["grade IS NOT NULL"]
end