title: See if value of String field is found in an Array
body: <p>I'm a little bit stuck with a query in mongodb and I think I am just looking for it wrong. 
  What I am trying to do is run a query which returns all documents where a value for a specific field is somewhere in an Array. </p>
  
  <p>It's not <code>$in</code>, because this checks an Array field for a specific value. I am trying to do the opposite. I want to do the operation on a String field and look inside an Array. I hope you understand what I mean, but I think it will become clear with an example.</p>
  
  <p>Example:</p>
  
  <p><div class="snippet" data-lang="js" data-hide="false" data-console="true" data-babel="false">
  <div class="snippet-code">
  <pre class="snippet-code-js lang-js prettyprint-override"><code>{
    hero: "The Flash",
    name: "Barry Allen",
    city: "Central City"
  },
  {
    hero: "Green Arrow",
    name: "Oliver Queen",
    city: "Star City"
  },
  {
    hero: "Batman",
    name: "Bruce Wayne",
    city: "Gotham City"
  }</code></pre>
  </div>
  </div>
  </p>
  
  <p>So assume I want all the heroes active in both <strong>Gotham City</strong> and <strong>Central City</strong>. I want do a query where city is in <code>["Gotham City", "Central City"]</code>.</p>