When calling #last on a context, the view within the context is modified to have the reverse _id sorting. The same is not true in #first, and I think this should be changed.

https://github.com/mongodb/mongoid/blob/064293d2de71ed5d368f789ff2b239495d305027/lib/mongoid/contextual/mongo.rb#L674

This is causing calling #first after #last to behave unexpectedly:

context.first # first doc
context.last # last doc
context.first # last doc

Additionally #last attempts to get the sort from the criteria, and not from the view, like first does. I think this should be made consistent. Note that when creating a context from a criteria that has a sort, the sort is also transferred to the view.

We have two options:
1) change #first to get the sort from the criteria
2) change #last to not modify the view