How to get data from 2 tables?

I want to get the ids of respondents who have not answered any questions. How do I accomplish this?

Below are my models and their relationships to each other.

Answer model (fields: id, inquiry_id, text):

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class Answer < ActiveRecord::Base
  belongs_to :inquiry
  belongs_to :question
  has_one    :respondent,   :through => :inquiry

  validates_uniqueness_of :inquiry_id
  validates_presence_of   :text
end

Respondent model (fields: id, email, user_id):

class Respondent < ActiveRecord::Base
  has_many :inquiries, :dependent => :destroy
  has_many :questions, :through => :inquiries
  belongs_to :user

  validates_uniqueness_of :email
  validates_presence_of :email
end

Inquiry model (fields: id, question_id, respondent_id):

class Inquiry < ActiveRecord::Base
  belongs_to  :question
  belongs_to  :respondent
  has_one    :answer, :dependent => :destroy

Question model (fields: id, text):

class Question < ActiveRecord::Base
  has_many :inquiries, :dependent => :destroy
  has_many :answers, :through => :inquiries, :dependent => :destroy
  belongs_to :user
end

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Depending on the context in which you want this data, and assuming you dont have too many Respondents and Questions you could do the following. This assumes you have a method called answered? in your question model that returns true if the question is answered.

respondents = [] 
Respondent.all.each do |respondent|

  total_questions = respondent.questions.count
  answered_questions = 0

  unless total_questions < 1   
    respondent.questions.each do |q|
      q.answered? ? answered_questions++ : nil
    end
  end

  if answered_questions == 0
    respondents << respondent
  end
end

At the end you are left with a array of respondent objects, meaning you can iterate over it to get the ids.