This is my trick: keep the latest group info and do update only for one document in collection.
And if we don't specify any sort the index is used by default and that one, which includes fields in query. In most cases we use `groupIdQuery` and it specifies `GROUP_ID` field. As far as we have only one index which starts with it, that one is selected to get a document for select and update - performance, though! 
And that's why I added an important para in docs: to avoid priority/non-priority indexes ambiguity.
If you wish I can add comments here.