Getting error "uninitialized constant Rake::RDocTask" while running rake db:migrate

I am upgrading an existing rails application from Rails verison 2.3.5 to 3.0.3.
I added some files, and while running rake db:migrate, getting error:

uninitialized constant Rake::RDocTask

After long searching, running rake --trace and analyzing found the fix.
Added the line

require 'rake/testtask'

require 'rake/rdoctask'

below "require 'rake'" in Rakefile and it worked.