Use a global variable in read_preference context manager if outside o…

…f request context.

flask_mongoengine/context_managers.py

@@ -1,13 +1,30 @@
-from flask import g
+from flask import g, has_request_context
 from mongoengine.queryset.queryset import QuerySet
 
 orig_cursor_args = QuerySet._cursor_args.fget
 
+# In case we're not in a request context
+_read_preference = None
+
+def _get_read_preference():
+    if has_request_context():
+        return getattr(g, 'read_preference', None)
+    else:
+        return _read_preference
+
+def _set_read_preference(val):
+    if has_request_context():
+        g.read_preference = val
+    else:
+        _read_preference = val
+
+
 def _patched_cursor_args(self):
     cursor_args = orig_cursor_args(self)
-    if getattr(g, 'read_preference', None) is not None:
+    read_preference = _get_read_preference()
+    if read_preference is not None:
         if not 'read_preference' in cursor_args:
-            cursor_args['read_preference'] = g.read_preference
+            cursor_args['read_preference'] = read_preference
             del cursor_args['slave_okay']
     return cursor_args
 
@@ -18,7 +35,7 @@ def __init__(self, read_preference):
         self.read_preference = read_preference
 
     def __enter__(self):
-        g.read_preference = self.read_preference
+        _set_read_preference(self.read_preference)
 
     def __exit__(self, t, value, traceback):
-        g.read_preference = None
+        _set_read_preference(None)