title: Find documents with array field that only contains specific values and nothing else
body: <p>Consider this collection:</p>
  <pre><code>{
    { name: 'first', toys: ['doll', 'car', 'doll'],
    { name: 'second', toys: ['doll', 'car'],
    { name: 'third', toys: ['doll', 'car', 'bricks'],
    { name: 'fourth', toys: ['doll', 'bricks'],
    { name: 'fifth', toys: []
  }
  </code></pre>
  <p>I want to query for documents whose <code>toys</code> field is an array that only contains <code>doll</code> and <code>car</code>. In this case, both <code>first</code> and <code>second</code> should match. <code>first</code> matches because <code>doll</code> and <code>car</code> can be repeated in the array, <code>third</code> doesn't match because no other value must be present in the array, and <code>fourth</code> and <code>fifth</code> don't match because they don't contain both <code>doll</code> and <code>car</code>.</p>
  <p>Using <code>$all</code> and <code>$in</code> doesn't work for me because they match <code>third</code>. How can I achieve this? Thanks!</p>