db_conversion.rb

db = ActiveRecord::Base.connection

puts '#!/bin/bash'
puts ""
puts "COMMAND='dry-run'"
puts ""

db_conversion = "CHARACTER SET utf8mb4 COLLATE utf8mb4_unicode_ci;"
puts "pt-online-schema-change -uroot --alter '#{db_conversion}' D=#{db.current_database} --chunk-size=10k --critical-load Threads_running=200 --set-vars innodb_lock_wait_timeout=2 --alter-foreign-keys-method=auto --$COMMAND"

db.tables.each do |table|
  column_conversions = db.columns(table).map do |column|
    case column.sql_type
    when /([a-z])*text/i, /varchar\(([0-9]+)\)/i
      sql_type = column.sql_type.upcase
      default = (column.default.nil?) ? '' : "DEFAULT \"#{column.default}\""
      null = (column.null) ? '' : 'NOT NULL'
      "MODIFY \`#{column.name}\` #{sql_type} CHARACTER SET utf8mb4 COLLATE utf8mb4_unicode_ci #{default} #{null}".strip
    end
  end.compact

  puts "# #{table}"
  if column_conversions.empty?
    puts "# NO CONVERSIONS NECESSARY FOR #{table}"
  else
    table_conversion = "ROW_FORMAT=DYNAMIC CHARACTER SET utf8mb4 COLLATE utf8mb4_unicode_ci"
    puts "pt-online-schema-change -uroot --alter '#{table_conversion}' D=#{db.current_database},t=#{table} --chunk-size=10k --critical-load Threads_running=200 --set-vars innodb_lock_wait_timeout=2 --alter-foreign-keys-method=auto --$COMMAND"
    puts "pt-online-schema-change -uroot --alter '#{column_conversions.join(", ")}' D=#{db.current_database},t=#{table} --chunk-size=10k --critical-load Threads_running=200 --set-vars innodb_lock_wait_timeout=2 --alter-foreign-keys-method=auto --$COMMAND"
  end
  puts ""
end