ruby on rails - How can I write a down or reversible function in this migration? -

so, in-case migration causes errors i'd able revert back/rake db:rollback.

the code:

class changeuidtoprimarykey < activerecord::migration def change execute alter table "users" drop constraint "users_pkey". execute alter table "users" add primary key "uid" end end 

what down/reversible code can add make rake db:rollback work in case?

you following i.e. keeping unique index on email if need revert back.

class changeuidtoprimarykey < activerecord::migration   def self.up     execute "alter table users add constraint unique_users_email unique(email)"     execute "alter table users drop constraint users_pkey"     execute "alter table users add primary key (uid)"   end    def self.down     execute "alter table users drop constraint users_pkey"     execute "alter table users drop constraint unique_users_email"     execute "alter table users add primary key (email)"   end end