title: How to find and merge mongodb document
body: <p>I'm trying test application that should merge two documents with same key. I thought that findAndModify() will do a job, but it doesn't. For example, lets assume that there is a document in 'data' collection:
  { "Date":"2014-08-23", "field1":"A"}</p>
  
  <p>I tried:</p>
  
  <pre><code>bo.put( "Date", "2014-08-23");
  bo.put( "field2", "B" );
  query.put( "Date", "2014-08-23" );
  
  DBObject resObj = collection.findAndModify( query, bo );
  </code></pre>
  
  <p>The result is { "Date" : "2014-08-23", "field2" : "B" }
  And I want { "Date" : "2014-08-23", "field1":"A", "field2" : "B" }</p>
  
  <p>How I can do it within one DB or Collection command? </p>