ActiveRecord::Reflection
参考：
ActiveRecord::Reflection::ClassMethods | Ruby on Rails API [Official]
ActiveRecord::Reflection::MacroReflection | Ruby on Rails API [Official]
rails/reflection.rb at v5.2.3 · rails/rails – GitHub
reflections
参考：
reflections – ActiveRecord::Reflection::ClassMethods | Ruby on Rails API [Official]
How to get activerecord associations via reflection – Stack Overflow
reflect_on_association
参考：
reflect_on_association – ActiveRecord::Reflection::ClassMethods | Ruby on Rails API [Official]
reflect_on_all_associations
参考：
モデル間のアソシエーションの情報を取得する方法 | present (tnakamura.hatenablog.com)
reflect_on_all_associations – ActiveRecord::Reflection::ClassMethods | Ruby on Rails API [Official]
Is there a way to list all belongs_to associations? – Stack Overflow