Ruby on Rails

class Answer < ActiveRecord::Base
  belongs_to :reply
  belongs_to :question
  belongs_to :possible_answer
end

poll.rb

class Poll < ActiveRecord::Base
  validates_presence_of :title

  has_many :questions
  has_many :replies
end

possible_answer.rb

class PossibleAnswer < ActiveRecord::Base
  belongs_to :question
end

question.rb

class Question < ActiveRecord::Base
  belongs_to :poll

  has_many :possible_answers
  has_many :answers

  accepts_nested_attributes_for :possible_answers, reject_if: proc { |attributes| attributes['title'].blank? }
end

reply.rb

class Reply < ActiveRecord::Base
  belongs_to :poll
  has_many :answers

  accepts_nested_attributes_for :answers
end


In the views I have a reply/new.html.erb that already work for radio and open answer questions, by rendering the partial by kind:

<h1><%= @poll.title %></h1>

<%= form_for [ @poll, @reply ] do |f| %>
    <%= f.fields_for :answers do |c| %>
        <%= render c.object.question.kind, c: c %>
    <% end %>
    <p>
      <%=f.submit 'Finish poll', class: 'btn btn-primary'%>
    </p>
<% end %>

and the partial for the checkbox:

<p>
  <%= c.label :value, c.object.question.title %>
</p>
<div class="checkbox">
  <% c.object.question.possible_answers.each do |possible_answer| %>
      <p>
        <label>
          <%= check_box_tag( 'possible_answer_id['+ possible_answer.id.to_s+']', possible_answer.id) %>
          <%= possible_answer.title %>
          <%= c.hidden_field :question_id %>
        </label>
      </p>
  <% end %>
</div>

and the partial for the radio buttons:

<p>
  <%= c.label :value, c.object.question.title %>
</p>
<div class="radio">
  <% c.object.question.possible_answers.each do |possible_answer| %>
      <p>
        <label>
          <%= c.radio_button :possible_answer_id, possible_answer.id %>
          <%= possible_answer.title %>
          <%= c.hidden_field :question_id %>
        </label>
      </p>
  <% end %>
</div>

This is my data base model:

for now i can reply to a poll and answer to the 3 kind of questions but the checkboxes kind won't save to the answers table
Probably I have to use has_many through association in the answers model but I'm not getting how. Can someone help me?
Thanks!