title: Find and update nested array
body: <p>My JSON object as shown below and I want to update one product details of user 1
  by using User id and product id</p>
  
  <pre><code>{
     "_id": {
     "$oid": "5b544ee53379672e184c9aa2"
  },
  "user_firstname": "user 1",
  "user_lastname": "test123",
  "products": [
     {
       "_id": {
       "$oid": "5b5615741026382bf0b784a6"
      },
       "product_name": "product 1",
       "product_price": {
       "$numberInt": "25"
       },
     },
     {
       "_id": {
       "$oid": "5b5615ba1026382bf0b784a7"
      },
      "product_name": "product 2",
      "product_price": {
      "$numberInt": "25"
      },
     },
   ]
   }{
      "_id": {
        "$oid": "5b574e38b714e420683ae839"
       },
      "user_firstname": "user2",
     "user_lastname": "user last name",
     "products": [
       {
          "_id": {
          "$oid": "5b574e7fb714e420683ae83b"
       },
        "product_name": "product1",
        "product_price": {
        "$numberInt": "10"
       },
     },
     ],
    }
  </code></pre>
  
  <p>Here two users are there and they have some products. Now I want to update one product details of particular user by finding his ID. In the above example I want to update <strong>"$oid": "5b5615ba1026382bf0b784a7"</strong> product details of the user with id <strong>"$oid": "5b544ee53379672e184c9aa2"</strong></p>
  
  <p>This details I will get from the user</p>
  
  <pre><code>updateproducts =   {
             product_id : "5b5615ba1026382bf0b784a7"
             product_name : "updated product"
             product_price :"250"
             userID : "5b544ee53379672e184c9aa2"
          }
  </code></pre>
  
  <p>And this is my function</p>
  
  <pre><code>  module.exports.updateProducts = function(updateproducts, callback) {
       var id = updateproducts.product_id;
       sellers.update({ "_id": ObjectId(id) }, { '$set': updateproducts }, 
  callback);
    }
  </code></pre>