Rails 4 dynamic form nested attributes without hidden input

Question

Previously I asked a question about building an attendance list getting the students and building a check list to mark attendance and came up with this.

def new
    @attendance_list = @classroom.attendance_lists.new

    @attendance_list.attendances = @classroom.student_ids.map do |student_id|
      @attendance_list.attendances.build(student_id: student_id)
    end
end

def create
    @attendance_list = @classroom.attendance_lists.new(attendance_list_params)
    @attendance_list.classroom_id = params[:classroom_id]

    respond_to do |format|

end

params

params.require(:attendance_list)
  .permit(:post_date, :remark, 
          attendances_attributes: [:student_id, :attended, :remarks ])

with simple fields

= simple_form_for [@school, @classroom, @attendance_list] do |f|
  = f.input :post_date
  = f.input :remark

    = f.simple_fields_for :attendances do |g|
      ** you see i needed a hidden student_id **
      = g.input :student_id, as: :hidden
      ...... 

model

class AttendanceList < ActiveRecord::Base
  belongs_to :classroom
  has_many :attendances
  has_many :students, :through => :attendances

  accepts_nested_attributes_for :attendances
end

class Attendance < ActiveRecord::Base
  belongs_to :student
  belongs_to :attendance_list
end

class Classroom < ActiveRecord::Base
  belongs_to :school
  has_and_belongs_to_many :students
  has_many :attendance_lists

  validates :class_name, presence: true
end

how do I do without the hidden input because this line doesn't seem to work.

build(student_id: student_id)

Answer 1

class AttendanceList < ActiveRecord::Base
  belongs_to :classroom
  has_many :attendances
  has_many :students, :through => :attendances
  accepts_nested_attributes_for :attendances
end

class Attendance < ActiveRecord::Base
  belongs_to :student
  belongs_to :attendance_list
end

class Classroom < ActiveRecord::Base
  belongs_to :school
  has_and_belongs_to_many :students
  has_many :attendance_lists

  validates :class_name, presence: true
end

class Student < ActiveRecord::Base
  has_many :attendances
  has_many :attendance_lists, :through => :attendances
end

view file

= simple_form_for @student do |f|
  = f.simple_fields_for :attendances do |g|
      ** your code **