dereference instance not thread-safe

AUTHORS

@@ -172,3 +172,4 @@ that much better:
  * Alon Horev (https://github.com/alonho)
  * Kelvin Hammond (https://github.com/kelvinhammond)
  * Jatin- (https://github.com/jatin-)
+ * Thom Knowles (https://github.com/fleat)

mongoengine/base/fields.py

@@ -186,7 +186,6 @@ class ComplexBaseField(BaseField):
     """
 
     field = None
-    __dereference = False
 
     def __get__(self, instance, owner):
         """Descriptor to automatically dereference references.
@@ -201,9 +200,11 @@ class ComplexBaseField(BaseField):
                        (self.field is None or isinstance(self.field,
                         (GenericReferenceField, ReferenceField))))
 
+        _dereference = _import_class("DeReference")()
+
         self._auto_dereference = instance._fields[self.name]._auto_dereference
-        if not self.__dereference and instance._initialised and dereference:
-            instance._data[self.name] = self._dereference(
+        if instance._initialised and dereference:
+            instance._data[self.name] = _dereference(
                 instance._data.get(self.name), max_depth=1, instance=instance,
                 name=self.name
             )
@@ -222,7 +223,7 @@ class ComplexBaseField(BaseField):
         if (self._auto_dereference and instance._initialised and
            isinstance(value, (BaseList, BaseDict))
            and not value._dereferenced):
-            value = self._dereference(
+            value = _dereference(
                 value, max_depth=1, instance=instance, name=self.name
             )
             value._dereferenced = True
@@ -382,13 +383,6 @@ class ComplexBaseField(BaseField):
 
     owner_document = property(_get_owner_document, _set_owner_document)
 
-    @property
-    def _dereference(self,):
-        if not self.__dereference:
-            DeReference = _import_class("DeReference")
-            self.__dereference = DeReference()  # Cached
-        return self.__dereference
-
 
 class ObjectIdField(BaseField):
     """A field wrapper around MongoDB's ObjectIds.