title: Rails query all records whose all associated record has nil or some specific value in some specific fields
body: <p>I have two models with a <code>has_many</code> and <code>belongs_to</code> relations:</p>
  
  <pre><code>class Parent &lt; ActiveRecord::Base
    has_many :children
  end
  
  class Child &lt; ActiveRecord::Base
    belongs_to :parent
  end
  </code></pre>
  
  <p>How do I find all those Parents whose all children have nil value in some specific field.</p>