Nested attrivutes ; :reject_if (Rails 2.3.4)

Hi There!

I got a strange behavior while playin' with nested attributes.I want
to reject a creation if e.g. "attr" is blank, through a model (one-to-

I tried this code with no success.
:reject_if => proc { |a| a[:attr].blank? } (symbol hash key)

This code runs correctly.
:reject_if => proc { |a| a['attr'].blank? } (string hash key)

I thought this is the same? I saw many example with an symbol hash...

Thanks a lot...