has_many :through in Rails with TB2

fields_for.rb

# --- model
class Announcement < ActiveRecord::Base
  belongs_to :user

  has_many :announcement_rooms, dependent: :delete_all
  has_many :rooms, through: :announcement_rooms
end


# --- controller
  def new
    result = run Announcement::Create::Present
    render html: cell(Announcement::Cell::Create, @form,
                      context: default_context(),
                      layout: School::Cell::Layout)
  end

  def create
    run Announcement::Create do |op|
      return redirect_to(teacher_announcements_path)
    end
    render html: cell(Announcement::Cell::Create, @form,
                      context: default_context(),
                      layout: School::Cell::Layout)
  end
# --- contract
require "reform/form/dry"
module Announcement::Contract
  class Create < Reform::Form
    feature Reform::Form::Dry

    property :text
    validation :default do
      required(:text).filled
    end
    

    collection :rooms,   
      skip_if: lambda { |fragment, *| fragment["selected"] == 'false' },
      prepopulator: ->(options) { 
          self.rooms = Room.all 
      }, 
      populator: ->(fragment:, **) {
        unless item = Room.find_by(id: fragment["id"])
          return skip!
        end        

        if fragment["selected"] == "false"
          rooms.delete(item)
          return skip!
        end

        rooms.append(item)        
      } do
      property :id
      property :name
      property :set, virtual: true
  end
    
    
# ---Operation
class Announcement::Create < Trailblazer::Operation
	class Present < Trailblazer::Operation
	  step  Model( Announcement, :new )
	  step  Policy::Pundit( AnnouncementPolicy, :create? )
	  step  :assign_current_user!
	  step  Contract::Build( constant: Announcement::Contract::Create )
	  step  :prepopulate!
	  def prepopulate!(options)
	    options["contract.default"].prepopulate!
	  end

	  def assign_current_user!(options)
	    options["model"].user =  options["current_user"]
	  end
	end	

  step  Nested( Present )
  step  Contract::Validate(key: "announcement")
  failure  Contract::Persist(method: :sync)
  step  Contract::Persist()
end
    
#  ---- create.slim  (form)    
h3 Anons yaratma
= cell(School::Cell::Errors, model)
= form_for [:teacher, model], html: {class: 'ui form'} do |f|
  .field
    label
      | Anons
    = f.text_area :text

  .inline.fields
    label for='role' Sinifi   
    - f.fields_for :rooms do |room|   
      .field
        .ui.checkbox.checkbox 
          = room.hidden_field :id
          = room.check_box :selected, {checked: announcement.rooms.collect(&:id).include?(room.object.id)}, "true", "false"
          label = room.object.name
  input class="ui button" type="submit" value='Kaydet'
  

• I want to show rooms as check boxes

But when validation fails after form submit, it is duplicating selected room checkboxes on the form.