Rails AR/Firebird replace current BigDecimal to BigNum for columntype BigInt

hello list,

i dont know where i can change it, but fireruby maps the columntype
BigInt of a firebird 2.0 database
to BigDecimal. generated ids will displayed with as a floating point
number
thats a bad display.

can anybody tell me, where i can define which ruby datatype is used by
the given column type?

many thanks

best regards