Handle another case where a blob might be erroneously purged

activestorage/lib/active_storage/attached/one.rb

@@ -64,20 +64,25 @@ def purge_later
 
     private
       def replace(attachable)
-        unless attachable == blob
-          previous_blob = blob
+        blob_was = blob
+        blob = create_blob_from(attachable)
 
+        unless blob == blob_was
           transaction do
             detach
-            write_attachment build_attachment_from(attachable)
+            write_attachment build_attachment(blob: blob)
           end
 
-          previous_blob.purge_later
+          blob_was.purge_later
         end
       end
 
       def build_attachment_from(attachable)
-        ActiveStorage::Attachment.new(record: record, name: name, blob: create_blob_from(attachable))
+        build_attachment blob: create_blob_from(attachable)
+      end
+
+      def build_attachment(blob:)
+        ActiveStorage::Attachment.new(record: record, name: name, blob: blob)
       end
 
       def write_attachment(attachment)

activestorage/test/models/attachments_test.rb

@@ -68,6 +68,18 @@ class ActiveStorage::AttachmentsTest < ActiveSupport::TestCase
     end
   end
 
+  test "replaced attached blob with itself by signed ID" do
+    @user.avatar.attach create_blob(filename: "funky.jpg")
+
+    assert_no_changes -> { @user.reload.avatar.blob } do
+      assert_no_changes -> { @user.reload.avatar.attachment } do
+        assert_no_enqueued_jobs do
+          @user.avatar.attach @user.avatar.blob.signed_id
+        end
+      end
+    end
+  end
+
   test "attach blob to new record" do
     user = User.new(name: "Jason")