title: MongoDB update all fields of array error
body: <p>Im tring to set 0 the items.qty of a document obtains by a id query.</p>
  
  <pre><code>db.warehouses.update(
      // query 
      {
          _id:ObjectId('5322f07e139cdd7e31178b78')
      },    
      // update 
      {
          $set:{"items.$.qty":0}
      },    
      // options 
      {
          "multi" : true,  // update only one document 
          "upsert" : true  // insert a new document, if no existing document match the query 
      }
  );
  </code></pre>
  
  <p>Return:</p>
  
  <p>Cannot apply the positional operator without a corresponding query field containing an array.</p>
  
  <p>This is the document that i want to set all items.qty to 0</p>
  
  <pre><code>{
    "_id": { "$oid" : "5322f07e139cdd7e31178b78" },
    "items": [
      {
        "_id": { "$oid" : "531ed4cae604d3d30df8e2ca" },
        "brand": "BJFE",
        "color": "GDRNCCD",
        "hand": 1,
        "model": 0,
        "price": 500,
        "qty": 0,
        "type": 0
      },
      {
        "brand": "BJFE",
        "color": "GDRNCCD",
        "hand": 1,
        "id": "23",
        "model": 0,
        "price": 500,
        "qty": 4,
        "type": 0
      },
      {
        "brand": "BJFE",
        "color": "GDRNCCD",
        "hand": 1,
        "id": "3344",
        "model": 0,
        "price": 500,
        "qty": 6,
        "type": 0
      }
    ],
    "name": "a"
  }
  </code></pre>