GORM get/find resource by ID using MongoDB in Grails

When a domain object is persisted in MongoDB, it is stored as a document with an ObjectId as a unique 12 bytes BSON primary key. For example, if you have a domain object Product like

import org.bson.types.ObjectIdclass Product {    ObjectId id    String name    static mapWith = "mongo"}

then the persisted entity in MongoDB would look like below if you save with a name as "TestProduct".

//db.product.find() in mongodb{   "_id" : ObjectId("51bd047c892c8bf0b3a58b21"),   "name" : "TestProduct",   "version" : 0 }

The _id becomes your primary key for that document. To get this document you need the primary key ObjectId. Speaking from a RESTful context, you atleast need the 12 bytes hexcode 51bd047c892c8bf0b3a58b21 as part of the request.

So in the above case, you can fetch that particular document by doing something like

Product.get(new ObjectId("51bd047c892c8bf0b3a58b21"))Product.findById(new ObjectId("51bd047c892c8bf0b3a58b21"))

Have a look at the API for ObjectId which would make clear how to retrieve a document.

When you retrieve the document as JSON, then it just shows the ObjectId class with its elements.

{    "class": "com.example.Product",    "id": {        "class": "org.bson.types.ObjectId",        "inc": -1280996575,        "machine": -1993569296,        "new": false,        "time": 1371341948000,        "timeSecond": 1371341948    },    "name": "TestProduct"}

For completeness, here's the domain with a controller to get a resource by ID string (instead of ObjectID).

Example:
Metric.get("51bf8ccc30040460f5a05579")

Domain

import org.bson.types.ObjectIdclass Metric {   ObjectId id   String   title   static mapWith = "mongo"   def out() {      return [         id:    id as String,  //convert Mongo ObjectId to 12-byte hex BSON         title: title         ]      }   }

The out() method is used to render the resource showing its ID string (not its ObjectID).

Controller

import grails.converters.JSONclass MetricController {   def index() {      def resource = Metric.read(params.id)      render resource.out() as JSON      }   }

Example JSON response for /rest/metric/51bf8ccc30040460f5a05579

{ "id": "51bf8ccc30040460f5a05579", "title": "Accounts" }