Idempotent Rails Migrations for Columns

add_environment_to_host.rb

 class AddEnvironmentToHost < ActiveRecord::Migration
     def self.up
-        add_column :hosts, :environment, :string
+        unless ActiveRecord::Base.connection.columns(:hosts).collect {|c| c.name}.include?("environment")
+            add_column :hosts, :environment, :string
+        end
     end
     
     def self.down
-        remove_column :hosts, :environment
+        if ActiveRecord::Base.connection.columns(:hosts).collect {|c| c.name}.include?("environment")
+            remove_column :hosts, :environment
+        end
     end
 end

Shamelessly purloined from https://projects.puppetlabs.com/issues/1959