The bug of mongoid returning first document when invoking last?

That's not a bug in Mongoid, that's a bug in your expectations of the first and last methods. From the fine version 5 manual:

#first ⇒ Document

Note: Mongoid previously added an _id sort when no sort parameters were provided explicitly by the user. This caused bad performance issues and was not expected, so #first/#last will no longer guarantee order if no sorting parameters are provided. For order guarantees - a sort must be explicitly provided.

Get the first document in the database for the criteria's selector.

So first (as of Mongoid5) gives you the first document in the query with respect to the current order but it no longer supplies a default order. You'll need to supply your own sort order in the query if you want first to behave like it used to.

Similarly for last.

It seems the old behaviour is back in Mongoid 6? - I'm using 6.1.0 and I'm experiencing the following:

t(prod)> Benchmark.measure { Event.where(:created_at => 7.days.ago..1.day.ago).first }=> #<Benchmark::Tms:0x00000003cb00b8 @cstime=0.0, @cutime=0.0, @label="", @real=4.109985898248851, @stime=0.0, @total=0.0, @utime=0.0>t(prod)> Benchmark.measure { Event.where(:created_at => 7.days.ago..1.day.ago).limit(1).entries.first }=> #<Benchmark::Tms:0x000000037377f8 @cstime=0.0, @cutime=0.0, @label="", @real=2.6328365616500378, @stime=0.0, @total=0.0, @utime=0.0>

So using .first instead of .limit(1).entries.first is about twice as slow. One workaround is to do: .first(id_sort: :none)

hope that helps :)