title: mongodb's $inc, incrementing a field that does not exists (upsert case)
body: <p>What happens when you increment a field that does not exist (upsert case) ?...</p>
  
  <pre><code>db.collection.findAndModify({query:{id:"id_in_param"},
                               update:{$inc:{score:1}},
                               upsert:true
                             })
  </code></pre>