backport https://github.com/rails/rails/issues/4651 to rails 3.2

config-initializers-hotfixes.rb

if Rails::VERSION::MAJOR > 3 || Rails::VERSION::MINOR > 2
  raise "Get rid of line #{__LINE__} in #{__FILE__}"
else
  require 'schema_hotfix'
end

lib-schema_hotfix.rb

require 'active_record/connection_adapters/abstract_mysql_adapter'

ActiveRecord::ConnectionAdapters::AbstractMysqlAdapter.class_eval do
  def add_index_length(option_strings, column_names, options = {})
    if options.is_a?(Hash) && length = options[:length]
      case length
      when Hash
        column_names.each {|name| option_strings[name] += "(#{length[name]})" if length.has_key?(name) && length[name].present?}
      when Fixnum
        column_names.each {|name| option_strings[name] += "(#{length})"}
      end
    end

    return option_strings
  end
end