Rails API with Grape, polymorphic: true, touch: true is not working

Question

I am developing a Rails 4.2.6 API app with a number of polymorphic associations, the problem is touch: true does not show the updated object.

class Post < ActiveRecord::Base
  belongs_to :postable, polymorphic: true, touch: true
end

class Event < ActiveRecord::Base
  has_many :posts, as: :postable
end

class Agenda < ActiveRecord::Base
  belongs_to :event, touch: true
  has_many :sessions, through: :days, class_name: 'AgendaSession'
end

class AgendaDay < ActiveRecord::Base
  belongs_to :agenda, touch: true
  has_many :sessions, class_name: 'AgendaSession'
  delegate :event, to: :agenda
end

class AgendaSession < ActiveRecord::Base
  belongs_to :agenda_day, touch: true
  delegate :event, to: :agenda_day
  has_many :posts, as: :postable
end

I am trying to retrieve the Event's attribute 'hashtag' through Post.postable. For the sake of argument lets say I have an event with hashtag: '#rock_events' and a session that belongs to the same event.

@event_post = Event.first.posts.first
@session_post = AgendaSession.first.posts.first

# Through console (and rspec):

@event_post.postable.hashtag
>> '#rock_events'
@session_post.postable.event.hashtag
>> '#rock_events'

# Edit the event's hashtag:

Event.first.update(hashtag: '#pop_event')
>> true
@event_post.postable.hashtag
>> '#rock_events'
@session_post.postable.event.hashtag
>> '#rock_events'

My current dirty fix is:

class Post < ActiveRecord::Base
  belongs_to :postable, polymorphic: true, touch: true
  belongs_to :event, foreign_key: :postable_id, touch: true
  belongs_to :session, class_name: 'AgendaSession', foreign_key: :postable_id, touch: true
end

# and to call a post's parent's hashtag:

@event_post.postable.posts.find(@event_post.id).event.hashtag
@session_post.postable.posts.find(@session_post.id).session.event.hashtag

I understand that I'm reloading all associations in order to get the updated event's hashtag. Anyone knows of a fix or a better work around for this predicament?

Thank You