If you are merging two hashes, like I was, and you want to make sure AR respects the items you are specifying, then specify all your hashes with string keys. i.e., say this:

prepopulated_fields = { ‘address’ => ’100 main st.’, ‘city’ => ‘Pasadena’ }
build_payment(prepopulated_fields.merge!(params[:payment]))

That way, if params[:payment] has a nil ‘address’, it will get overridden by your ‘address’ because they have the same key. The problem only occurs when one of the sources has a key ‘address’ (string) and the other has a key :address (symbol), which allows both to exist in the hash simultaneously.