Ignore deleted files in Active Storage migration

Files might be missing for whatever reason or records might not point to
any files; in these edge cases, we were getting an exception.

lib/tasks/active_storage.rake

@@ -40,7 +40,11 @@ namespace :active_storage do
 
         model.find_each.each do |instance|
           attachments.each do |attachment|
-            next if instance.send(attachment).path.blank?
+            source = instance.send(attachment).path
+
+            next if source.blank?
+
+            file = File.read(source) if File.exist?(source)
 
             connection.exec_prepared(
               statement_name, [
@@ -48,7 +52,7 @@ namespace :active_storage do
                 instance.send("#{attachment}_file_name"),
                 instance.send("#{attachment}_content_type"),
                 instance.send("#{attachment}_file_size"),
-                Digest::MD5.base64digest(File.read(instance.send(attachment).path)),
+                file && Digest::MD5.base64digest(file) || SecureRandom.hex(32),
                 instance.updated_at.iso8601,
                 "storage_#{attachment}",
                 model.name,
@@ -65,9 +69,11 @@ namespace :active_storage do
       name = attachment.name.delete_prefix("storage_")
       source = attachment.record.send(name).path
 
+      if source && File.exist?(source)
         FileUtils.mkdir_p(File.dirname(dest))
         logger.info "Copying #{source} to #{dest}"
         FileUtils.cp(source, dest)
       end
     end
+  end
 end

spec/lib/tasks/active_storage_spec.rb

@@ -29,6 +29,21 @@ describe "active storage tasks" do
       expect(storage_file_path(document)).to eq test_storage_file_paths.first
     end
 
+    it "migrates records with deleted files ignoring the files" do
+      document = create(:document,
+                        attachment: nil,
+                        paperclip_attachment: File.new("spec/fixtures/files/clippy.pdf"))
+      FileUtils.rm(document.attachment.path)
+
+      run_rake_task
+      document.reload
+
+      expect(ActiveStorage::Attachment.count).to eq 1
+      expect(ActiveStorage::Blob.count).to eq 1
+      expect(document.storage_attachment.filename).to eq "clippy.pdf"
+      expect(test_storage_file_paths.count).to eq 0
+    end
+
     def test_storage_file_paths
       Dir.glob("#{storage_root}/**/*").select { |file_or_folder| File.file?(file_or_folder) }
     end