1) Require rake and load the rake files containing the tasks and dependencies you want to run:
require ‘rake’
load File.dirname(__FILE__) + ‘/../vendor/rails/railties/lib/tasks/misc.rake’
load File.dirname(__FILE__) + ‘/../vendor/rails/railties/lib/tasks/databases.rake’
2) invoke the task
Rake::Task[”db:test:prepare”].invoke