You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: add_column.rb
    path of file: ./repos/zero_downtime_migrations/lib/zero_downtime_migrations/validation
    
    the code of the file until where you have to start completion: module ZeroDowntimeMigrations
  class Validation
    class AddColumn < Validation
      def validate!
 
    
      def message
        <<-MESSAGE.strip_heredoc
          Adding a column with a default is unsafe!

          This can take a long time with significant database
          size or traffic and lock your table!

          First let’s add the column without a default. When we add
          a column with a default it has to lock the table while it
          performs an UPDATE for ALL rows to set this new default.

            class Add#{column_title}To#{table_title} < ActiveRecord::Migration
              def change
                add_column :#{table}, :#{column}, :#{column_type}
              end
            end