title: Select the value from an array of hashes based on key from a single document in mongoid
body: <p>I have a document in <code>items</code> collection like </p>
  
  <pre><code>// Document One
  {
      "_id" : ObjectId("556411af73616d0d822f0000"),
      "visibility" : [
          {
              "user_id" : ObjectId("556412bb73616d0d82310000"),
              "visible" : false
          },
          {
              "user_id" : ObjectId("556412c973616d0d82320000"),
              "visible" : true
          }
      ]
  },
  
  // Some other documents
  {...}
  {...}
  </code></pre>
  
  <p>I want to get the value of <code>visible</code> only for "Document One" based on <code>user_id</code> I provide (eg. ObjectId("556412bb73616d0d82310000")).</p>
  
  <p>How?</p>
  
  <p>I am using mongoid 4.0.0.</p>
  
  <p>Thanks.</p>