DeReference is now used in a thread safe manner

No global / module instance is needed
Fixes #399

mongoengine/base.py

@@ -234,8 +234,8 @@ class ComplexBaseField(BaseField):
             return self
 
         if not self._dereference and instance._initialised:
-            from dereference import dereference
-            self._dereference = dereference  # Cached
+            from dereference import DeReference
+            self._dereference = DeReference()  # Cached
             instance._data[self.name] = self._dereference(
                 instance._data.get(self.name), max_depth=1, instance=instance,
                 name=self.name

mongoengine/dereference.py

@@ -186,5 +186,3 @@ class DeReference(object):
             return BaseDict(data, instance, name)
         depth += 1
         return data
-
-dereference = DeReference()

mongoengine/document.py

@@ -277,8 +277,8 @@ class Document(BaseDocument):
 
         .. versionadded:: 0.5
         """
-        from dereference import dereference
-        self._data = dereference(self._data, max_depth)
+        from dereference import DeReference
+        self._data = DeReference()(self._data, max_depth)
         return self
 
     def reload(self, max_depth=1):

mongoengine/queryset.py

@@ -1095,8 +1095,8 @@ class QuerySet(object):
         .. versionadded:: 0.4
         .. versionchanged:: 0.5 - Fixed handling references
         """
-        from dereference import dereference
-        return dereference(self._cursor.distinct(field), 1)
+        from dereference import DeReference
+        return DeReference()(self._cursor.distinct(field), 1)
 
     def only(self, *fields):
         """Load only a subset of this document's fields. ::
@@ -1747,10 +1747,10 @@ class QuerySet(object):
 
         .. versionadded:: 0.5
         """
-        from dereference import dereference
+        from dereference import DeReference
         # Make select related work the same for querysets
         max_depth += 1
-        return dereference(self, max_depth=max_depth)
+        return DeReference()(self, max_depth=max_depth)
 
 
 class QuerySetManager(object):