升级到Rails 6后，无效的多态关系和验证错误

Question

我有一个无法解决的错误。

我有这些课程

class Content < ApplicationRecord
  has_many                :product_relations, as:         :productable
  has_many                :products,          through:    :product_relations
end 

class Product < ApplicationRecord
  has_many :product_relations
  has_many :contents, through: :product_relations, source_type: "Content", source: :productable
end 

class ProductRelation < ApplicationRecord
  belongs_to :product
  belongs_to :content
  belongs_to :productable, polymorphic: true
end

Rails 6升级后，我出现此错误：

pry(#<Admin::ContentsController>)> content.errors
=> #<ActiveModel::Errors:0x00007ff0c8ff4770
 @base=
  #<Review:0x00007ff0cec5f6e0
   id: 35057,
   name: "pippo",
   type: "Review",
   permalink: "pippo",
   published_at: Mon, 01 Jun 2020 14:56:00 CEST +02:00,
   body: "<p>sss</p>",
   summary: "test",
   aasm_state: "published",
   author_id: 5,
   created_at: Wed, 06 May 2020 17:54:26 CEST +02:00,
   updated_at: Mon, 01 Jun 2020 15:00:08 CEST +02:00,
   source_link: "",
   source_name: "",
   square_crop_img: nil,
   impressions_count: 0,
   brand_contacts_notified_at: nil,
   main_image: "e571cc_server2.jpg",
   b_f_notified_at: nil,
   p_f_notified_at: nil,
   follows_count: 1,
   template: "large",
   tsv: "'pipp':1 'sss':2",
   pinned: false,
   not_in_home: false,
   sponsored: false,
   no_adv: false,
   presentation: "standard",
   comments_count: 0,
   brands_dday_companies_contacts_notified_at: nil,
   brands_dday_companies_companies_notifiable_ids: [],
   additional_html: "",
   dmove: false,
   tag_list: nil>,
 @details={:product_relations=>[{:error=>:invalid}, {:error=>:invalid}, {:error=>:invalid}, {:error=>:invalid}, {:error=>:invalid}, {:error=>:invalid}]},
 @messages={:product_relations=>["non è valido"]}>

基本上，我在内容验证错误。内容必须存在

"ActiveRecord::RecordInvalid: Validazione fallita: Content deve esistere
from /Users/scrivoaroby/.rbenv/versions/2.6.3/lib/ruby/gems/2.6.0/gems/activerecord-6.0.2.2/lib/active_record/validations.rb:81:in `raise_validation_error'

我不明白为什么