title: MongoDB Embedded Objects have no ID (null value)
body: <p>I have a question regarding MongoDB with Spring Data. 
  I have these domain classes:</p>
  
  <pre><code>@Document
  public class Deal  {
      @Id
      private ObjectId _id;
      private Location location;
      private User user;
      private String description;
      private String title;
      private String price;
      private boolean approved;
      private Date expirationDate;
      private Date publishedDate;
  }
  
  @Document
  public class Location {
      @Id
      private ObjectId _id;
      private Double latitude;
      private Double longitude;
      private String country;
      private String street;
      private String zip;
  }
  
  @Document
  public class User {
      @Id
      private ObjectId _id;
      private String email;
      private String password;
      private String profile_image_url;
      private Collection&lt;Deal&gt; deals = new ArrayList&lt;Deal&gt;();
  }
  </code></pre>
  
  <p>With these domains I can successfully CRUD. There is only one problem. When saving a User with Deals, the deals and Location get _id set to null when saving them to MongoDB.
  Why can´t MongoDB generate unique id´s for embedded objects?  </p>
  
  <p>The result after saving a User with one deal:</p>
  
  <pre><code>{ "_id" : ObjectId( "4fed0591d17011868cf9c982" ),
    "_class" : "User",
    "email" : "milo@gmail.com",
    "password" : "mimi",
    "deals" : [ 
      { "_id" : null,
        "location" : { "_id" : null,
          "latitude" : 2.22,
          "longitude" : 3.23445,
          "country" : "Denmark",
          "street" : "Denmark road 77",
          "zip" : "2933" },
        "description" : "The new Nexus 7 Tablet. A 7 inch tablet from Google.",
        "title" : "Nexus 7",
        "price" : "1300",
        "approved" : false,
        "expirationDate" : Date( 1343512800000 ),
        "publishedDate" : Date( 1340933521374 ) } ] }
  </code></pre>
  
  <p>As you can see from the result, Deal and Location ID is set to NULL.</p>