Tested on Mongoid 3.1.4:

class Band
include Mongoid::Document
has_many :albums
end

class Album
include Mongoid::Document
belongs_to :band
field :name
default_scope where(name: 'foo')
end

Album.create(band_id: Band.create.id, name: 'foo')
Album.create(band_id: Band.create.id, name: 'foo')

Band.includes(:albums).each do |band|
p band.albums.first.name # Does not hit the database again.
end

MOPED: 127.0.0.1:27017 QUERY database=gocheap_development collection=bands selector={} flags=[:slave_ok] limit=0 skip=0 batch_size=nil fields=nil (0.3586ms)
MOPED: 127.0.0.1:27017 QUERY database=gocheap_development collection=albums selector={"name"=>"foo", "band_id"=>{"$in"=>["51b58e5c5b9e053267000001", "51b58e5c5b9e053267000003"]}} flags=[:slave_ok] limit=0 skip=0 batch_size=nil fields=nil (0.3092ms)
MOPED: 127.0.0.1:27017 QUERY database=gocheap_development collection=albums selector={"$query"=>

{"name"=>"foo", "band_id"=>"51b58e5c5b9e053267000001"}

, "$orderby"=>{:_id=>1}} flags=[:slave_ok] limit=-1 skip=0 batch_size=nil fields=nil (0.3133ms)
"foo"
MOPED: 127.0.0.1:27017 QUERY database=gocheap_development collection=albums selector={"$query"=>

{"name"=>"foo", "band_id"=>"51b58e5c5b9e053267000003"}

, "$orderby"=>{:_id=>1}} flags=[:slave_ok] limit=-1 skip=0 batch_size=nil fields=nil (0.3765ms)
"foo"

So despite the fact that the query for 'includes' is done with the default_scope selector, the collected results are not used later.