Adding a check for exposable json attributes.

src/Jenssegers/Mongodb/Model.php

@@ -28,6 +28,14 @@ abstract class Model extends \Jenssegers\Eloquent\Model {
      */
     protected $primaryKey = '_id';
 
+	/**
+	 * Allow json attributes to be exposable. This is mainly for
+	 * relations that can be kept alive in a toJson()
+	 *
+	 * @var array
+	 */
+	protected $exposeJsonAttributes = [];
+
     /**
      * The connection resolver instance.
      *
@@ -284,7 +292,7 @@ abstract class Model extends \Jenssegers\Eloquent\Model {
             // internal array of embedded documents. In that case, we need
             // to hide these from the output so that the relation-based
             // attribute can take over.
-            else if (starts_with($key, '_'))
+            else if (starts_with($key, '_') and ! in_array($key, $this->exposeJsonAttributes))
             {
                 $camelKey = camel_case($key);