Rails 6 AssociationTypeMismatch с fields_for

Question

Не знаю, почему это не работает? Пожалуйста, помогите.

Rails 6

class Tjedan < ApplicationRecord
 has_many :termini
 accepts_nested_attributes_for :termini
end

class Termin < ApplicationRecord
 belongs_to :tjedan
 belongs_to :barber
 belongs_to :pon, class_name: 'Smjena', foreign_key: 'pon'
 belongs_to :uto, class_name: 'Smjena', foreign_key: 'uto'

end

class Smjena < ApplicationRecord
 has_many :barberi
 has_many :termini
end

Контроллер имеет

params.require(:tjedan).permit(:status, :start_date, termini_attributes: [:id, :pon, :uto])

, а форма заполняется

= form_with(model: [:admin, tjedan], local: true) do |f|
    .....
   = f.fields_for :termini do |t|   
    = t.collection_select(:pon, Smjena.all, :id, :ime, {selected: t.object.pon.id} )

при обновлении я получаю это параметры

Parameters: {
  "authenticity_token" => "HM2amibKuWBUlODuPVaKXZA9TzMYi2UTvrS38KlBwuazcRALhYwqhwaki5jB3go4CD8i+10vAI1PfTRX91xOQA==",
  "tjedan" => {
    "termini_attributes" => {
      "0" => {
        "pon" => "7", "id" => "1"
      }, "1" => {
        "pon" => "7", "id" => "2"
      }, "2" => {
        "pon" => "7", "id" => "3"
      }, "3" => {
        "pon" => "7", "id" => "4"
      }, "4" => {
        "pon" => "6", "id" => "5"
      }, "5" => {
        "pon" => "6", "id" => "6"
      }
    }
  },
  "commit" => "Submit",
  "id" => "21"
}

Тогда я получаю эту ошибку

ActiveRecord :: AssociationTypeMismatch в Admin :: TjedniController # update Ожидается Smjena (# 70142612753520), получил "7", который является экземпляром строки (# 70142566710760)