Using an eigenclass to make some instances of BSON::ObjectId to serialize to JSON as strings.

bson2json.rb

require 'json'
require 'bson'

a = BSON::ObjectId.new
b = BSON::ObjectId.new

puts a.to_json
# {"$oid":"5837b91f775ab9717f8d8bc9"}
puts b.to_json
# {"$oid":"5837b92c775ab9717f8d8bca"}

# Modify the "b" instance's eigenclass, eek?!
class << b
  def as_json
    to_s
  end
end

puts a.to_json
# {"$oid":"5837b91f775ab9717f8d8bc9"}
puts b.to_json
# "5837b92c775ab9717f8d8bca"