Trimmable lib removes leading and trailing white spaces from the AR attribute values.

sudo gem install trimmable

Run the following command in your Rails.root:

Rails 2.x
./script/plugin install [email protected]:viola/trimmable.git

Rails 3.x
./script/rails plugin install [email protected]:viola/trimmable.git

class Post < ActiveRecord::Base
  validates_presence_of :title
  trimmable_attributes :title, :comments
end

Started POST "/posts" for 127.0.0.1 at Sun Oct 31 20:30:59 -0400 2010
Processing by PostsController#create as HTML
Parameters: {"commit"=>"Create Post", "post"=>{"comments"=>"    new comment goes here      ", "title"=>"New title    "}, "authenticity_token"=>"Hjd4K8op5TAY7rL4k176tVehIr6eHzuR0vtNIRMlfRU=", "utf8"=>"✓"}
SQL (17.3ms)  INSERT INTO "posts" ("comments", "created_at", "title", "updated_at") VALUES ('new comment goes here', '2010-11-01 00:30:59.163811', 'New title', '2010-11-01 00:30:59.163811')
Redirected to http://localhost:3000/posts/12
Completed 302 Found in 40ms