How does one validate a 'float' field?

I would like to validate the following conditions on a numeric field:

1) That the user did not enter a non-numeric string where he was
required to enter a number.
2) That the numeric value is zero or greater.

There seems to be a conflict between these two requirements:

If the user enters a non-numeric string (e.g "ABC"), by the time it
reaches the validation function within the ActiveRecord object, it has
already been marshalled into a numeric format. "ABC" will be converted
to zero. According to condition #2, this is an acceptable value.
According to condition #1, it is not.

Is there anyway I can execute the validation methods on the raw string
values, before they are "casted" into the ActiveRecord types?
Otherwise, can you suggest an elegant solution for this problem?

Thanks!

Shimon Amit

Thank you. That is exactly what I was looking for.

Shimon Amit