models.rb

# app/models/list.rb
class List < AR::Base
  has_many :list_tasks
  has_many :tasks, through: :list_tasks
end

# app/models/task.rb
class Task < AR::Base
  has_many :list_tasks
  has_many :lists, through: :list_tasks
end

# app/models/list_task.rb
class ListTask < AR::Base
  belongs_to :list
  belongs_to :task
end