USING
    mongo_mapper 0.7.3
    bson 0.20.1
    mongo 0.20.1

>> Post.all
NoMethodError: undefined method `from_string' for Mongo::ObjectID:Class
        from /Library/Ruby/Gems/1.8/gems/mongo_mapper-0.7.3/lib/mongo_mapper/support.rb:135:in `to_mongo'
        from /Library/Ruby/Gems/1.8/gems/mongo_mapper-0.7.3/lib/mongo_mapper/plugins/keys.rb:329:in `set'
        from /Library/Ruby/Gems/1.8/gems/mongo_mapper-0.7.3/lib/mongo_mapper/plugins/keys.rb:290:in `write_key'
        from /Library/Ruby/Gems/1.8/gems/mongo_mapper-0.7.3/lib/mongo_mapper/plugins/dirty.rb:104:in `write_key'
        from (eval):10:in `_id='
        from /Library/Ruby/Gems/1.8/gems/mongo_mapper-0.7.3/lib/mongo_mapper/plugins/keys.rb:174:in `send'
        from /Library/Ruby/Gems/1.8/gems/mongo_mapper-0.7.3/lib/mongo_mapper/plugins/keys.rb:174:in `attributes='
        from /Library/Ruby/Gems/1.8/gems/bson-0.20.1/lib/../lib/bson/ordered_hash.rb:69:in `each_pair'
        from /Library/Ruby/Gems/1.8/gems/bson-0.20.1/lib/../lib/bson/ordered_hash.rb:69:in `each'
        from /Library/Ruby/Gems/1.8/gems/bson-0.20.1/lib/../lib/bson/ordered_hash.rb:69:in `each_pair'
        from /Library/Ruby/Gems/1.8/gems/mongo_mapper-0.7.3/lib/mongo_mapper/plugins/keys.rb:170:in `attributes='
        from /Library/Ruby/Gems/1.8/gems/mongo_mapper-0.7.3/lib/mongo_mapper/plugins/keys.rb:154:in `initialize'
        from /Library/Ruby/Gems/1.8/gems/mongo_mapper-0.7.3/lib/mongo_mapper/plugins/dirty.rb:42:in `initialize'
        from /Library/Ruby/Gems/1.8/gems/mongo_mapper-0.7.3/lib/mongo_mapper/plugins/keys.rb:56:in `new'
        from /Library/Ruby/Gems/1.8/gems/mongo_mapper-0.7.3/lib/mongo_mapper/plugins/keys.rb:56:in `load'
        from /Library/Ruby/Gems/1.8/gems/mongo_mapper-0.7.3/lib/mongo_mapper/document.rb:202:in `find_many'
        from /Library/Ruby/Gems/1.8/gems/mongo_mapper-0.7.3/lib/mongo_mapper/document.rb:201:in `map'
        from /Library/Ruby/Gems/1.8/gems/mongo_mapper-0.7.3/lib/mongo_mapper/document.rb:201:in `find_many'
        from /Library/Ruby/Gems/1.8/gems/mongo_mapper-0.7.3/lib/mongo_mapper/document.rb:100:in `all'