Belongs_to through association

class Speech < ApplicationRecord
  belongs_to :agenda_item, -> { includes :meeting }
  delegate :meeting, to: :agenda_item
  delegate :room, to: :meeting

class AgendaItem < ApplicationRecord
  belongs_to :meeting
  has_many :speeches, -> { order 'starting_at ASC' }, dependent: :destroy
  delegate :room, to: :meeting # this create a belongs_to room for agenda_item

class Meeting < ApplicationRecord
  belongs_to :room
  has_many :agenda_items, -> { order 'starting_at ASC' }, dependent: :destroy
  has_many :speeches, through: :agenda_items

class SpeechSerializer < ActiveModel::Serializer
  attributes :id, :order_num, :starting_at, :finishing_at
  belongs_to :agenda_item
  belongs_to :meeting
  belongs_to :room
end

Meeting.last.speeches
Room.last.meetings.map(&:speeches)

Speech.last.meeting
Speech.last.room