ActiveStorageを使用したモデルに関連するRequest SpecでActiveSupport::MessageVerifier::InvalidSignatureが発生する問題の解決策 ref: https://qiita.com/h-sakano/items/fc297f91a7bafc0b4d6d

file3.diff

-    attached { fixture_file_upload Rails.root.join('spec', 'fixtures', 'files', 'test.jpg'), 'image/jpg' }
+    
+    after(:build) do |hoge|
+      hoge.attached = fixture_file_upload(Rails.root.join('spec', 'fixtures', 'files', 'test.jpg'), 'image/jpg')
+    end

hoge.rb

class Hoge < ApplicationRecord
  .
  .
  .
  has_one_attached :attached
  .
  .
  .
end

hoge_factory.rb

FactoryBot.define do
  factory :hoge do
    name { Faker::Name.name }
    .
    .
    .
    attached { fixture_file_upload(Rails.root.join('spec', 'fixtures', 'files', 'test.jpg'), 'image/jpg') }
    .
    .
    .
    trait :edit do
      name "HOGE"
    end
  end
end

hogehoge_factory.rb

FactoryBot.define do
  factory :hoge do
    name { Faker::Name.name }
    .
    .
    .
    attached { fixture_file_upload(Rails.root.join('spec', 'fixtures', 'files', 'test.jpg'), 'image/jpg') }
    .
    .
    .
    trait :edit do
      name "HOGE"
    end
  end
end

hogehoges_spec.rb

require 'rails_helper'

RSpec.describe "Hogehoges", type: :request do
  .
  .
  .
  describe 'PUT #update' do
    let!(:hoge) { create(:hoge) }

    context 'when sign_in admin user' do
      before do
        sign_in @admin_user
      end

      it 'can update name' do
        # テストが失敗したので、例外の内容を出力してみると、「ActiveSupport::MessageVerifier::InvalidSignature」だった
        expect do
          put bad_report_url bad_report, params: { bad_report: attributes_for(:bad_report, :edit) }
        end.to change { BadReport.find(hoge.id).name }.from(hoge.name).to("HOGE")
      end
    end
  end
  .
  .
  .
end

hoges_spec.rb

require 'rails_helper'

RSpec.describe "Hoges", type: :request do
  .
  .
  .
  describe 'PUT #update' do
    let!(:hoge) { create(:hoge) }

    context 'when sign_in admin user' do
      before do
        sign_in @admin_user
      end

      it 'can update name' do
        # テストが失敗したので、例外の内容を出力してみると、「ActiveSupport::MessageVerifier::InvalidSignature」だった
        expect do
          put bad_report_url bad_report, params: { bad_report: attributes_for(:bad_report, :edit) }
        end.to change { BadReport.find(hoge.id).name }.from(hoge.name).to("HOGE")
      end
    end
  end
  .
  .
  .
end