title: Searching for value of any field in MongoDB without explicitly naming it
body: <p>I looked through the MongoDB documentation and googled this question but couldn't really find a suitable answer. So, here is what I'm looking for. 
  Assume I have a collection with elements like this:</p>
  
  <pre><code>{
     "foo" : "bar",
     "test" : "test",
     "key" : "value",
  }
  </code></pre>
  
  <p>What I'd like to achieve is find an element by searching in all (maybe except for finitely many ;-) ) fields. In other words: Given a query, I do NOT know in which field the query should be found.</p>
  
  <p>In my thinking something like this </p>
  
  <pre><code>db.things.find({_ANY_ : "bar"}) 
  </code></pre>
  
  <p>would give me the example element.</p>
  
  <p>Thank you for your help.</p>