I'm programming an application that saves its data in a MySQL
database. I frequently run into headaches where I get errors like:

    undefined method `-' for "9000.00":String (NameError)

because I typed

    source['salary'] -= encumbered['salary']

where source[] and encumbered[] were instantiated from
MysqlResult#fetch_hash, and are thus strings instead of integers or
floats.

I write this sort of thing all the time in Perl and don't have trouble
because of its automatic type conversion. What would be the best way
to do this in Ruby? Right now I'm writing:

    source['salary'] = source['salary'].to_f -
encumbered['salary'].to_f

everywhere and it feels quite tedious.