class Semester < ApplicationRecord
  has_and_belongs_to_many :students
  accepts_nested_attributes_for :students
end

class Student < ApplicationRecord
  has_and_belongs_to_many :semesters
  has_many :attendances, dependent: :destroy
  accepts_nested_attributes_for :attendances
end

class Attendance < ApplicationRecord
  belongs_to :semester
  belongs_to :student
  validates_presence_of :date
end

@semester.students.each do |student|
  student.attendances
end

# semesters_controller.rb
def show
  @semester = Semester.includes(students: [:attendances])
                      .order('students.first_name')
                      .find params[:id]
end

# students_helper.rb
def student_attendance(student)
  total = student.attendances.select { |x| x.semester_id == @semester.id }
  present = total.select &:present
  percent = (present.size/total.size.to_f * 100).round rescue 0
  link_to student, class: 'attendance', style: "width: #{percent}%" do
    <<-HTML.html_safe
      <span>#{student.first_name}</span>
      <span>#{percent}%</span>
    HTML
  end
end