Fix for FileField losing ref without default

fixes hmarr/mongoengine#458

docs/changelog.rst

@@ -4,6 +4,7 @@ Changelog
 
 Changes in 0.6.X
 ================
+- Fixed FileField losing reference when no default set
 - Removed possible race condition from FileField (grid_file)
 - Added assignment to save, can now do: b = MyDoc(**kwargs).save()
 - Added support for pull operations on nested EmbeddedDocuments

mongoengine/fields.py

@@ -946,14 +946,16 @@ class FileField(BaseField):
 
         # Check if a file already exists for this model
         grid_file = instance._data.get(self.name)
-        if isinstance(grid_file, self.proxy_class):
-            if not grid_file.key:
-                grid_file.key = self.name
-                grid_file.instance = instance
-            return grid_file
-        return self.proxy_class(key=self.name, instance=instance,
-                                db_alias=self.db_alias,
-                                collection_name=self.collection_name)
+        if not isinstance(grid_file, self.proxy_class):
+            grid_file = self.proxy_class(key=self.name, instance=instance,
+                                         db_alias=self.db_alias,
+                                         collection_name=self.collection_name)
+            instance._data[self.name] = grid_file
+
+        if not grid_file.key:
+            grid_file.key = self.name
+            grid_file.instance = instance
+        return grid_file
 
     def __set__(self, instance, value):
         key = self.name

tests/fields.py

@@ -3,6 +3,8 @@ import os
 import unittest
 import uuid
 import StringIO
+import tempfile
+import gridfs
 
 from decimal import Decimal
 
@@ -19,6 +21,10 @@ class FieldTest(unittest.TestCase):
         connect(db='mongoenginetest')
         self.db = get_db()
 
+    def tearDown(self):
+        self.db.drop_collection('fs.files')
+        self.db.drop_collection('fs.chunks')
+
     def test_default_values(self):
         """Ensure that default field values are used when creating a document.
         """
@@ -1647,6 +1653,48 @@ class FieldTest(unittest.TestCase):
         testimage.delete()
         self.assertFalse(testimage_fs.exists(testimage_grid_id))
 
+    def test_file_field_no_default(self):
+
+        class GridDocument(Document):
+            the_file = FileField()
+
+        GridDocument.drop_collection()
+
+        with tempfile.TemporaryFile() as f:
+            f.write("Hello World!")
+            f.flush()
+
+            # Test without default
+            doc_a = GridDocument()
+            doc_a.save()
+
+
+            doc_b = GridDocument.objects.with_id(doc_a.id)
+            doc_b.the_file.replace(f, filename='doc_b')
+            doc_b.save()
+            self.assertNotEquals(doc_b.the_file.grid_id, None)
+
+            # Test it matches
+            doc_c = GridDocument.objects.with_id(doc_b.id)
+            self.assertEquals(doc_b.the_file.grid_id, doc_c.the_file.grid_id)
+
+            # Test with default
+            doc_d = GridDocument(the_file='')
+            doc_d.save()
+
+            doc_e = GridDocument.objects.with_id(doc_d.id)
+            self.assertEquals(doc_d.the_file.grid_id, doc_e.the_file.grid_id)
+
+            doc_e.the_file.replace(f, filename='doc_e')
+            doc_e.save()
+
+            doc_f = GridDocument.objects.with_id(doc_e.id)
+            self.assertEquals(doc_e.the_file.grid_id, doc_f.the_file.grid_id)
+
+        db = GridDocument._get_db()
+        grid_fs = gridfs.GridFS(db)
+        self.assertEquals(['doc_b', 'doc_e'], grid_fs.list())
+
     def test_file_uniqueness(self):
         """Ensure that each instance of a FileField is unique
         """