_form.haml

= f.fields_for :documents_attributes do |ff|
  = ff.hidden_field :graphic, class: "upload-hidden"

Processing by Admin::LessonsController#create as HTML
  Parameters: {"utf8"=>"✓", "authenticity_token"=>"I3NfaupPeavweIfWD3bxxXpev3+46prtirGNxx88Fs/FGWBXuVO8GFu4qpFYxJFVugxhEIpFpd9amPbOgYIffg==", "lesson"=>{"title"=>"lesson 1", "video_link"=>"lesson 1", "documents_attributes"=>{"graphic"=>""}, "description"=>""}, "uploading_image"=>"", "commit"=>"Create Lesson", "course_id"=>"foobar"}
  Membership Load (0.3ms)  SELECT  "memberships".* FROM "memberships" WHERE "memberships"."id" = $1 LIMIT $2  [["id", 51], ["LIMIT", 1]]
  Course Load (0.3ms)  SELECT  "courses".* FROM "courses" WHERE "courses"."slug" = $1 LIMIT $2  [["slug", "foobar"], ["LIMIT", 1]]
Completed 500 Internal Server Error in 11ms (ActiveRecord: 0.6ms)



TypeError (no implicit conversion of Symbol into Integer):

app/controllers/admin/lessons_controller.rb:18:in `create'

Using Shrine Gem

document.rb

# == Schema Information
#
# Table name: documents
#
#  id                :bigint(8)        not null, primary key
#  graphic_data      :text
#  documentable_type :string
#  documentable_id   :bigint(8)
#  created_at        :datetime         not null
#  updated_at        :datetime         not null
#

class Document < ApplicationRecord
  belongs_to :documentable, polymorphic: true, optional: true
  include FileUploader::Attachment.new(:graphic)
end

file_uploader.rb

class FileUploader < Shrine
  plugin :activerecord
  plugin :cached_attachment_data # for retaining the cached file across form redisplays
  plugin :restore_cached_data # re-extract metadata when attaching a cached file
  plugin :determine_mime_type
  plugin :remove_attachment
  plugin :delete_raw
end

lessons_controller.rb

def lesson_params
 params.require(:lesson).permit(:video_link, :title, :slug, :description, documents_attributes: [:graphic])
end