This bug exists only in Mongoid 3 (testing on 3.0.15). Mongoid 2 doesn't have the bug.

foo = City.includes(:country).limit(3)
foo.first
MOPED: 127.0.0.1:27017 COMMAND database=admin command={:ismaster=>1} (1.5016ms)
MOPED: 127.0.0.1:27017 QUERY database=gocheap_development collection=places selector={"$query"=>{"deleted_at"=>nil, "_type"=>{"$in"=>["City"]}}, "$orderby"=>{:_id=>1}} flags=[:slave_ok] limit=-1 skip=0 fields=nil (5.1830ms)
MOPED: 127.0.0.1:27017 QUERY database=gocheap_development collection=places selector={"deleted_at"=>nil, "_id"=>

{"$in"=>["4eda5f2c8792904be40000fc"]}

, "_type"=>{"$in"=>["Country"]}} flags=[:slave_ok] limit=0 skip=0 fields=nil (0.6833ms)

So it selects just one country for the first city instead of selecting all the three countries.

bar = foo.to_a
MOPED: 127.0.0.1:27017 QUERY database=gocheap_development collection=places selector={"$query"=>{"deleted_at"=>nil, "_type"=>{"$in"=>["City"]}}, "$orderby"=>{:_id=>1}} flags=[:slave_ok] limit=3 skip=0 fields=nil (4.3747ms)
MOPED: 127.0.0.1:27017 KILL_CURSORS cursor_ids=[9172683583138442393] (0.1442ms)

bar.last.country
MOPED: 127.0.0.1:27017 QUERY database=gocheap_development collection=places selector={"$query"=>{"deleted_at"=>nil, "_id"=>"4eda5f2c8792904be40000c7", "_type"=>{"$in"=>["Country"]}}, "$orderby"=>{:_id=>1}} flags=[:slave_ok] limit=-1 skip=0 fields=nil (0.8352ms)

So all the other cities have to make separate requests for their countries as the Criteria is already affected by 'first'.