The controller:

class \WeblogController < ActionController::Base
  def show # not used directly, just example
    @post = Post.find(@params["id"])
  end

  def new
    @post = Post.new
  end
  
  def create
    @post = Post.new(@params["post"])
    if @post.save
      redirect_to :action => "show", :id => @post.id
    else
      render_action "new"
    end
  end
end

The new template:

<% unless @post.errors.empty? %>
  The post couldn't be saved due to these errors:
  
  <% @post.errors.each_full do |message| %>
    <%= message %>
  <% end %>
  
<% end %>

<%= form "post" %>

or even shorter:


<%= error_messages_for 'post' %>

The model (0.9):

class Post < ActiveRecord::Base
    validates_presence_of :title, :body, :message => "Missing required field"
    validates_length_of :title, :maximum => 10, :message => "Title too long - max 10 characters"
end

The model (pre 0.9):

class Post < ActiveRecord::Base
  protected
    def validate
      errors.add_on_empty(["title", "body"])
      errors.add("title", "must be at most 10 characters") if title.size > 10
    end
end

The "Validation Documentation":http://rails.rubyonrails.com/classes/ActiveRecord/Validations.html has all sorts of details about what methods are available. Also check out the "validation class methods":http://rails.rubyonrails.com/classes/ActiveRecord/Validations/ClassMethods.html available in Rails 0.9. Note: _validates_confirmation_of_ is very weak. If the *_confirmation field is nil, the confirmation does not take place. This makes it easy for poorly written code to by pass the confirmation. category:Howto