Make ActiveRecord ignore columns, so you can drop them without downtime.

gistfile1.rb

ActiveRecord::Base.class_eval do
  def self.ignore_columns(*names)
    extend ColumnIgnorance
    ignored_columns.merge names.map(&:to_s)
  end
 
  module ColumnIgnorance
    def columns
      return @columns if @columns
      @columns = super.reject do |column|
        ignored_columns.include?(column.name)
      end
    end
 
    def ignored_columns
      @ignored_columns ||= Set[]
    end
  end
end