title: MongoDB querying on array of documents
body: <p>I have the following data in the db </p>
  
  <pre><code>  { item: "journal", instock: [ { warehouse: "A", qty: 5 }, { warehouse: "C", qty: 15 } ] },
    { item: "notebook", instock: [ { warehouse: "C", qty: 5 } ] },
    { item: "paper", instock: [ { warehouse: "A", qty: 60 }, { warehouse: "B", qty: 15 } ] },
    { item: "planner", instock: [ { warehouse: "A", qty: 40 }, { warehouse: "B", qty: 5 } ] },
    { item: "postcard", instock: [ { warehouse: "B", qty: 15 }, { warehouse: "C", qty: 35 } ] }
  </code></pre>
  
  <p>I need to find out the warehouse which has </p>
  
  <ul>
  <li><code>15</code> qty of <code>paper</code></li>
  <li><code>35</code> qty of <code>postcard</code></li>
  </ul>