I have a standalone app that runs on Rails 3.0.7, that is also used as a rails engine, which is run under rails 3.1.8 from the parent app. Now, I have a registration form which connects to a user model, has validations and so forth. When I was running the app as an engine, and I would try to register, I could see in the logs a "cannot mass-assign attributes" warning.
My user.rb model file has no calls to attr_accessible or attr_protected. Adding a call to attr_accessible for the given fields on that model fixed the issue. However, I am wondering what might be causing it. When I run the app as standalone, it works fine. I'm not sure if the rails version has anything to do with it, but it's an important difference to note.
check if parent projects config/environments/x.rb file has config.active_record.mass_assignment_sanitizer = :strict, if so commenting it may work but not advisable