title: Updating multiple documents in mongodb using _id field
body: <p>I have sample products table and would like to update multiple documents using _id field. Every time, I try this it only updates the first doc in the $in clause I mentioned , not updating all. </p>
  
  <pre><code>db.products.update({_id:{$in:[ObjectId("507d95d5719dbef170f15bff"),
  ObjectId("507d95d5719dbef170f15c01"), ObjectId("507d95d5719dbef170f15c00")]}},
  {$set:{'monthly_price':7865}}, {multi:true})
  </code></pre>