classes.rb

class Questionnaire < ApplicationRecord
  has_many :questions
  has_many :member_questionnaires
  has_many :users, through: :member_questionnaires
  accepts_nested_attributes_for :questions
end


class Question < ApplicationRecord
  belongs_to :questionnaire
  has_one :questionnaire_answer
  accepts_nested_attributes_for :questionnaire_answer
end

class QuestionnaireAnswer < ApplicationRecord
  belongs_to :question
end

new.haml

%h1
  questionnaires#edit

= form_for @answer do |f|
  - @questionnaire.questions.each.with_index do |q, index|
    %h1
      = q.description
    = f.fields_for :questions, q do |question_fields|
      = question_fields.fields_for :questionnaire_answer, q.build_questionnaire_answer do |answer_fields|
        = answer_fields.text_field :description, id: index

  = f.submit "Save answers"

params

{                                                                  
                    "utf8" => "✓",                                 
    "questionnaire_answer" => {                                    
        "questions" => {                                           
            "questionnaire_answers" => {                           
                "description" => "Wayfarers park pug ennui letterpress irony."                                                         
            }                                                      
        }                                                          
    },                                                             
                  "commit" => "Save answers",                      
              "controller" => "questionnaire_answers",             
                  "action" => "create"                             
}