Added a failing test to reproduce issue #47.

tests/mongodb/models.py

@@ -1,6 +1,7 @@
 from django.db import models
-from djangotoolbox.fields import RawField
+from djangotoolbox.fields import RawField, ListField, EmbeddedModelField
 from django_mongodb_engine.fields import GridFSField, GridFSString
+from query.models import Post
 
 class DescendingIndexModel(models.Model):
     desc = models.IntegerField()
@@ -46,5 +47,8 @@ class GridFSFieldTestModel(models.Model):
     gridstring = GridFSString()
 
 
+class Issue47Model(models.Model):
+    foo = ListField(EmbeddedModelField(Post))
+
 RawModel.objects.all().delete()
 RawModel.objects.create(raw=42)

tests/mongodb/tests.py

@@ -100,6 +100,18 @@ def test_databasewrapper_api(self):
             for call in calls:
                 call()
 
+class RegressionTests(TestCase):
+    def test_issue_47(self):
+        """ ForeignKeys in subobjects should be ObjectIds, not unicode """
+        from bson.objectid import ObjectId
+        from query.models import Blog, Post
+        post = Post.objects.create(blog=Blog.objects.create())
+        m = Issue47Model.objects.create(foo=[post])
+        collection = get_collection(Issue47Model)
+        assert collection.count() == 1
+        doc = collection.find_one()
+        self.assertIsInstance(doc['foo'][0]['blog_id'], ObjectId)
+
 class DatabaseOptionTests(TestCase):
     """ Tests for MongoDB-specific database options """