TL;DR

oj 2.18.0 で This changes the API slightly. な変更が入った

Added a separate option for as_json instead of piggy backing on the use_to_json. This changes the API slightly.

to_json をcallするoptionがdefault false になった
to_json のcallを期待する場合は、都度指定するか defualt optionを設定するとよい

default option

require 'oj'

::Oj.default_options = {:mode => :compat, use_to_json: true }

::Oj.dump(value)

都度

require 'oj'

::Oj.dump(value, :mode => :compat, use_to_json: true)

Ref

https://github.com/ohler55/oj
:compatモードについて

:compat mode attempts to be compatible with other systems. It will serialize any Object, but will check to see if the Object implements an to_hash or to_json method. If either exists, that method is used for serializing the Object. Since as_json is more flexible and produces more consistent output, it is preferred over the to_json method. If neither the to_json or to_hash methods exists, then the Oj internal Object variable encoding is used.

use_to_jsonについて

:use_to_json [Boolean] call to_json() methods on dump, default is false