Added migration file

db/migrate/20131019023429_change_bike_shop_id.rb

@@ -0,0 +1,23 @@
+class ChangeBikeShopId < ActiveRecord::Migration
+  # We don't use change_column because we have arbitrary strings we
+  # need to strip the characters from to make the integer values
+  def up
+    bikes = Bike.all
+    remove_column :bikes, :shop_id
+    add_column :bikes, :shop_id, :integer, :unique => true
+
+    # deconflict shop ids
+    bikes.each do |bike|
+      new_id = bike.shop_id.gsub(/[^\d]/, '').to_i rescue 1
+      while bikes.map(&:shop_id).include? new_id
+        new_id += 1
+      end
+      bike.shop_id = new_id
+    end
+    bikes.each(&:save)
+  end
+
+  def down
+    change_column :bikes, :shop_id, :string
+  end
+end