title: MongoDB Morphia save() generates two objects with the same ID
body: <p>I have a java application that connects to a MongoDB Database through the Morphia library. My POJO that I store in the database has String field named _id and annotated with the @Id annotation (com.google.code.morphia.annotations.Id;).</p>
  
  <p>I'm generating a new object ( it has null _id). 
  I call save(object) on the datastore provided by morphia.
  The object gets updated after being stored and now has an _id value.
  I call save(object) again and a new entry is created in the database with the same _id.
  All consecutive save() operations on the object overwrite the old one and do not produce any new entries in the database. </p>
  
  <p>So for example, after 10 save() calls on the same object my database ends up looking like this:</p>
  
  <pre><code>{ "_id" : { "$oid" : "539ade7ee4b0451f28ba0e2e"} , "className" : "blabla" , blabla ...}
  { "_id" : "539ade7ee4b0451f28ba0e2e" , "className" : "blabla" , blabla ...}
  </code></pre>
  
  <p>As seen those two entries have the same _id but with different representation. One has it as an object the other as a string. Normally I should have only one entry shouldn't I ?</p>