title: Update and/or add array element properties using req.body via Mongoose?
body: <p>I have the following document:</p>
  
  <pre><code>{
      "_id" : ObjectId("503b83dfad79cc8d26000004"),
      "pdfs" : [
          {
              "title" : "Test document",
              "pdf_id" : ObjectId("504f6793ce351a595d000004"),
              "created_at" : ISODate("2012-09-11T16:32:19.276Z")
          },
          {
              "title" : "Some other doc",
              "pdf_id" : ObjectId("502bf124b4642341230003f0"),
              "created_at" : ISODate("2012-09-11T11:34:19.276Z")
          }
      ]
  }
  </code></pre>
  
  <p>Now in an incoming form via <code>req.body</code>, I have 2 fields: <code>title</code> and <code>description</code>. </p>
  
  <p>I want to update <code>title</code> and insert <code>description</code> for a specified pdf_id, how do I do that?</p>
  
  <p>So in the end, my document will now look like:</p>
  
  <pre><code>{
      "_id" : ObjectId("503b83dfad79cc8d26000004"),
      "pdfs" : [
          {
              "title" : "This is an UPDATED title",
              "description" : "It has an ALL NEW description",
              "pdf_id" : ObjectId("504f6793ce351a595d000004"),
              "created_at" : ISODate("2012-09-11T16:32:19.276Z")
          },
          {
              "title" : "Some other doc",
              "pdf_id" : ObjectId("502bf124b4642341230003f0"),
              "created_at" : ISODate("2012-09-11T11:34:19.276Z")
          }
      ]
  }
  </code></pre>
  
  <p>Just to be clear, I'm really just looking for the Mongoose <code>update</code> syntax.</p>