Q: is it possible to add migration for rails app to ruby gem? For my rails project I want to write a ruby gem which have an ActiveRecord model eg. Animal < ActiveRecord::Base. Is it possible to add migration cretae_animals to the gem in such a way that when the gem installed in my app and I run rake db:migrate that migration will execute? 

A: You can do this by including the migrations with your gem, as well as including a rake task that runs them. Then you call the rake task as follows:
myGem = Gem::Specification.find_by_name 'gem-name'
load "#{myGem.gem_dir}/lib/tasks/my_migration.rake"

(Proper credit to Andy Atkinson, where I originally learned this for a similar project.)
