title: MongoDB updating field contained in subdocument throws an error
body: <p>My collection schema looks like this:</p>
  
  <pre><code>Products :{ _id: "S2oF6HitSKzwy5toz",
              user_id: "LKAHKhK5SvTr6r8sK",
              shop: "eBay.com",
              items: {
                      item: "HP Laptop"
                      item_total: 101.67
                      notes: "note for hp laptop"
                      price: 101.67
                      quantity: 1
                      shop: "eBay.com"
                      url: "ebay.com/hp"
                      },
              expenses: {
                         bank_fee: 0
                         cod_fee: 0
                         expense_total: 0
                         inbound_fee: 0
                         pcs_fee: 0
                         pjp_fee: 0
                        },
              service_fee: 15.25,
              total: 116.92
             }
  </code></pre>
  
  <p>I want to update <code>bank_fee</code> field in the <code>expenses</code> subdocument using <code>update</code> query using this syntax:</p>
  
  <pre><code>insertFee : function(user_id, id, fee_amount){
      Products.update({user_id: user_id, _id: id}, {$set: {"expenses.$.bank_fee":fee_amount}});
  }
  </code></pre>
  
  <p>However it throws the error: </p>
  
  <pre><code>Exception while invoking method 'insertFee' MongoError: can't append to array us
  ing string field name [$] 
  </code></pre>
  
  <p>What I am doing wrong here?</p>
  
  <p>P.S. I am following <a href="http://docs.mongodb.org/manual/reference/operator/positional/" rel="nofollow">this doc</a> for update operation.</p>