Gemfile

source :rubygems

DM_VERSION = "~> 1.2.0"

gem "dm-core",           DM_VERSION
gem "dm-serializer",     DM_VERSION

to_json_bug.rb

require "dm-core"
require "dm-serializer"

DataMapper.setup(:default, :adapter => :in_memory)

class User
  include DataMapper::Resource

  property :id, Serial
end

DataMapper.finalize

3.times { User.create }

p User.all.class
#=> DataMapper::Collection

p DataMapper::Collection.instance_methods.grep /to_json/
#=> [:to_json]

p User.all.to_json
#=> undefined method `to_json' for [{:id=>1}, {:id=>2}, {:id=>3}]:Array (NoMethodError)