Cannot submit Checkbox through nested form - ruby-on-rails

Question

Im trying to create an application were teachers a able to select students each day who aren't in school. I created the models through nifty-generators gem. The problem is that it wont submit to the notpresents table. Please help.
# == Schema Information
#
# Table name: students
#
# id :integer not null, primary key
# name :string(255)
# group_id :integer
# created_at :datetime not null
# updated_at :datetime not null
#
class Student < ActiveRecord::Base
attr_accessible :name, :group_id
belongs_to :days
end
# == Schema Information
#
# Table name: notpresents
#
# id :integer not null, primary key
# student_id :integer
# day_id :integer
# created_at :datetime not null
# updated_at :datetime not null
#
class Notpresent < ActiveRecord::Base
attr_accessible :student_id, :day_id
belongs_to :days
end
# == Schema Information
#
# Table name: days
#
# id :integer not null, primary key
# title :string(255)
# created_at :datetime not null
# updated_at :datetime not null
#
class Day < ActiveRecord::Base
attr_accessible :title, :presents
has_many :notpresents
accepts_nested_attributes_for :notpresents
end
And view _form.html.erb
<%= form_for #day do |f| %>
<%= f.error_messages %>
<p>
<%= f.label :title %><br />
<%= f.text_field :title %>
</p>
<% for student in Student.find(:all) %>
<div>
<%= check_box_tag :notpresents, student.id%>
<%= student.name %>
</div>
<% end %>
<p><%= f.submit %></p>
<% end %>

Answer 1

I've never used the nifty-generators gem, but if a student can be absent in many days, and a day can have many students absent, shouldn't you have a many-to-many relationship?
class Student < ActiveRecord::Base
attr_accessible :name
has_many :days, through: :notpresents
has_many :notpresent
end
class Days < ActiveRecord::Base
attr_accessible :date
has_many :students, through: :notpresents
has_many :notpresent
end
class :Notpresents < ActiveRecord::Base
attr_accessible :student_id, :day_id
belongs_to :students
belongs_to :days
end
It could also be a has_and_belongs_to_many association, but with a has_many :through you could have a string or text attribute to make a note about the absence or something like that.
I recommend using simple_form for the form, it makes it so easy:
app/controllers/days_controller.rb:
def edit
#day = Day.find(params[:id])
end
app/views/days/_form.html.erb :
<%= simple_form_for #day do |f| %>
<%= f.association :students, as: :check_boxes %>
<% end %>