One of the first hurdles I ran into when I started with Rails migrations was:
undefined method define_a_column' for class OCI8::Cursor'
It means that your LD_LIBRARY_PATH environment variable does not include the Oracle client binaries.
(I got my clue on how to solve this issue from this old defect ticket on the Rails Trac).
Technorati Tags: Ruby, Rails, Oracle, OCI, Andrew Beacock