How to get the maximum length configured in an ActiveRecord validation?

More concise:

Person.validators_on(:name).detect { |v| v.is_a?(ActiveModel::Validations::LengthValidator) }.options[:maximum]

Uses detect{} instead of select{}.first and is_a? instead of class ==.
That works with Rails 4.1 as well.

Use validators_on method

irb(main):028:0> p Person.validators_on(:name)[0].options[:maximum]
50
=> 50

As @Max Williams mentioned it works only on Rails 3

The problem with @nash answer is that validators do not own a certain order. I figured out how to do the same thing with just some more code but in some kind of safer mode ('cause you can add more validators later and break the order you get it):

(Person.validators_on(:name).select { |v| v.class == ActiveModel::Validations::LengthValidator }).first.options[:maximum]

I think it does only work for Rails 3 too.

[Edit 2017-01-17] Carefull my answer is old (2012) and was for Rails 3. It may not work / be ideal for newer Rails versions.

---

Just to bring a little more DRY spirit, you could create a generic class method to get maximum "length_validator" value on any attribute, like so:

Create a module in your lib directory and make it extend ActiveSupport::Concern:

module ActiveRecord::CustomMethods
  extend ActiveSupport::Concern
end

# include the extension 
ActiveRecord::Base.send(:include, ActiveRecord::CustomMethods)

Add the "module ClassMethods" in it and create the "get_maximum" method:

module ActiveRecord::CustomMethods
  extend ActiveSupport::Concern

  module ClassMethods
    def get_maximum(attribute)
      validators_on(attribute).select{|v| v.class == ActiveModel::Validations::LengthValidator}.first.options[:maximum]
    end
  end
end

EDIT 1: Configuration

You'll also have to add a require in one of your initializers.

For instance, here are my configurations:

1. I've put my file in lib/modules/active_record/extensions.
2. I've added this in my autoload_paths: config.autoload_paths += %W(#{config.root}/lib/modules) Note: this is not required, but best practice if you want to put there some of your custom classes and modules that you share between your apps.
3. In one of my initializers (config/initializers/custom.rb) I've added this line: require "active_record/extensions"

And that should do it! Restart your server and then...

END EDIT 1

And then you should be able to do something like this:

<%= form_for @comment do |f| %>
  <%= f.text_area(:body, :maxlength => f.object.class.get_maximum(:body)) #Or just use Comment.get_maximum(:body) %>
<% end %>

I hope it will help others! :) Of course you can customize the method the way you want and add options and do fancy stuff. ;-)