example.rb

class User < ApplicationRecord
  has_many :user_todo_lists
  has_many :todo_lists, through: :user_todo_lists
  has_many :todos, through: :todo_lists
end

class UserTodoList < ApplicationRecord
  belongs_to :user
  belongs_to :todo_list
end

class TodoList < ApplicationRecord
  has_many :user_todo_lists
  has_many :users, through: :user_todo_lists
  has_many :todos
end

class Todo < ApplicationRecord
  belongs_to :todo_list
end