title: Find out if a model contains associations described in an array with ids
body: <p>I have the following models:</p>
  
  <pre class="lang-rb prettyprint-override"><code>class Person &lt; ApplicationRecord
      has_many :pets
  end
  
  class Pet &lt; ApplicationRecord
      belongs_to :person
  end
  </code></pre>
  
  <p>Now I have an array with the ids of certain pets, and I want to check if the person owns them all.</p>
  
  <pre><code>_pets = [1, 4, 5]
  
  person.owns(_pets)
  </code></pre>
  
  <p>So now I want to find out how to check if the user owns ALL these pets. These means that I want to find out if <code>_pets</code> is a subset of all his pets.</p>
  
  <pre class="lang-rb prettyprint-override"><code>class Person &lt; ApplicationRecord
      has_many :pets
  
      def owns(_pets)
          # ???
          # Returns true or false
      end
  end
  
  class Pet &lt; ApplicationRecord
      belongs_to :person
  end
  </code></pre>