title: Find record whose field 'name' not contained within any other record
body: <p>I have a model <code>Foo</code> with a <code>String</code> <code>bar</code> and a <code>String</code> <code>name</code>. Some records' <code>bar</code> contain the <code>name</code> of other records in them. This is intentional.</p>
  
  <p>I want to find the "root Foo" records - that is, the ones where their <code>name</code> do not appear in the <code>bar</code> records of any other <code>Foo</code> records.</p>
  
  <p>Example:</p>
  
  <pre><code>Foo
  id: 1
  name: 'foo1'
  bar:  'something something'
  
  id: 2
  name: 'foo2'
  bar:  'foo1 something'
  
  id: 3
  name: 'foo3'
  bar:  'foo1, foo4'
  </code></pre>
  
  <p>My method <code>root_foos</code> would return <code>foo2</code> and <code>foo3</code> since their names do not appear in any <code>bar</code> string.</p>
  
  <p>edit: I don't want to use a relation or foreign key here - just this method.</p>