java - MongoDB Embedded Objects have no ID (null value)

Question

I have a question regarding MongoDB with Spring Data. I have these domain classes:

@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<Deal> deals = new ArrayList<Deal>();
}

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?

The result after saving a User with one deal:

{ "_id" : ObjectId( "4fed0591d17011868cf9c982" ),
  "_class" : "User",
  "email" : "[email protected]",
  "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 ) } ] }

As you can see from the result, Deal and Location ID is set to NULL.

Answer 1

MongoDB CRUD operations (insert, update, find, remove) all operate on top-level documents exclusively -- although of course you can filter by fields in embedded documents. Embedded documents are always returned within the parent document.

The _id field is a required field of the parent document, and is typically not necessary or present in embedded documents. If you require a unique identifier, you can certainly create them, and you may use the _id field to store them if that is convenient for your code or your mental model; more typically, they are named after what they represent (e.g. "username", "otherSystemKey", etc). Neither MongoDB itself, nor any of the drivers will automatically populate an _id field except on the top-level document.

Specifically in Java, if you wish to generate ObjectId values for the _id field in embedded documents, you can do so with:

someEmbeddedDoc._id = new ObjectId();