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):

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

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.