Tested version - 3.1.2.

In the docs you state that this should work:

class Band
include Mongoid::Document
has_many :albums
end

class Album
include Mongoid::Document
belongs_to :band
end

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

So I did this:

class Band
include Mongoid::Document
has_many :albums
end

class Album
include Mongoid::Document
belongs_to :band
field :name
end

Album.create(band_id: Band.create.id)
Album.create(band_id: Band.create.id)

And then did this with logging:

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 (0.3102ms)
MOPED: 127.0.0.1:27017 QUERY database=gocheap_development collection=albums selector={"band_id"=>{"$in"=>["514acafe5b9e05fdec000001", "514acafe5b9e05fdec000003"]}} flags=[:slave_ok] limit=0 skip=0 batch_size=nil fields=nil (0.3438ms)
MOPED: 127.0.0.1:27017 QUERY database=gocheap_development collection=albums selector={"$query"=>

{"band_id"=>"514acafe5b9e05fdec000001"}

, "$orderby"=>{:_id=>1}} flags=[:slave_ok] limit=-1 skip=0 batch_size=nil fields=nil (0.5193ms)
MOPED: 127.0.0.1:27017 QUERY database=gocheap_development collection=albums selector={"$query"=>

{"band_id"=>"514acafe5b9e05fdec000003"}

, "$orderby"=>{:_id=>1}} flags=[:slave_ok] limit=-1 skip=0 batch_size=nil fields=nil (0.2167ms)

So it does hit the database again.