rake db:migrate not working for me

I am getting this error

 rake db:migrate (in
   /Users/john/Sites/my_app) rake aborted!
   Don't know how to build task
   'db:migrate'

When I trace the error I get

rake aborted! Don't know how to build task 'db:migrate' 
/usr/local/lib/ruby/gems/1.8/gems/rake-0.8.7/lib/rake.rb:1728:in `[]' 
/usr/local/lib/ruby/gems/1.8/gems/rake-0.8.7/lib/rake.rb:2050:in `invoke_task' 
/usr/local/lib/ruby/gems/1.8/gems/rake-0.8.7/lib/rake.rb:2029:in `top_level' 
/usr/local/lib/ruby/gems/1.8/gems/rake-0.8.7/lib/rake.rb:2029:in `each' 
/usr/local/lib/ruby/gems/1.8/gems/rake-0.8.7/lib/rake.rb:2029:in `top_level'
/usr/local/lib/ruby/gems/1.8/gems/rake-0.8.7/lib/rake.rb:2068:in `standard_exception_handling' 
/usr/local/lib/ruby/gems/1.8/gems/rake-0.8.7/lib/rake.rb:2023:in `top_level' 
/usr/local/lib/ruby/gems/1.8/gems/rake-0.8.7/lib/rake.rb:2001:in `run' 
/usr/local/lib/ruby/gems/1.8/gems/rake-0.8.7/lib/rake.rb:2068:in `standard_exception_handling' 
/usr/local/lib/ruby/gems/1.8/gems/rake-0.8.7/lib/rake.rb:1998:in `run' 
/usr/local/lib/ruby/gems/1.8/gems/rake-0.8.7/bin/rake:31 
/usr/local/bin/rake:19:in `load' /usr/local/bin/rake:19

My Rakefile is

require(File.join(File.dirname(__FILE__), 'config', 'boot'))

require 'rake'
require 'rake/testtask'
require 'rake/rdoctask'

For Ruby on Rails 2.3 you should have the following additional line at the end of your Rakefile:

require 'tasks/rails'