model 関連付けを調べる
全部
User.reflect_on_all_associations
belongs_to だけ
Group.reflect_on_all_associations(:belongs_to)
has_many だけ
Group.reflect_on_all_associations(:has_many)
User.reflect_on_all_associations
Group.reflect_on_all_associations(:belongs_to)
Group.reflect_on_all_associations(:has_many)