Ensure $maxDistance is always the last part of the query (#179)

mongoengine/queryset/transform.py

@@ -1,5 +1,7 @@
 from collections import defaultdict
 
+from bson import SON
+
 from mongoengine.common import _import_class
 from mongoengine.errors import InvalidQueryError, LookUpError
 
@@ -123,6 +125,16 @@ def query(_doc_cls=None, _field_operation=False, **query):
         elif key in mongo_query:
             if key in mongo_query and isinstance(mongo_query[key], dict):
                 mongo_query[key].update(value)
+                # $maxDistance needs to come last - convert to SON
+                if '$maxDistance' in mongo_query[key]:
+                    value_dict = mongo_query[key]
+                    value_son = SON()
+                    for k, v in value_dict.iteritems():
+                        if k == '$maxDistance':
+                            continue
+                        value_son[k] = v
+                    value_son['$maxDistance'] = value_dict['$maxDistance']
+                    mongo_query[key] = value_son
             else:
                 # Store for manually merging later
                 merge_query[key].append(value)