How to flash error for particular validation - ruby-on-rails-3

Question

I am trying to flash messages in a form when they do not meet the validation requirements but can't work out how to achieve this.
I have the following setup:
models/item.rb
class Item < ActiveRecord::Base
attr_accessible :condition, :day, :description, :subtitle, :title
validates :user_id, presence: true
validates :title, presence: true
validates :description, presence: true, length: { minimum: 20 }
belongs_to :user
end
controllers/items_controller.rb
class ItemsController < ApplicationController
before_filter :authenticate_user!
def new
#item = Item.new
end
def show
#item = Item.find(params[:id])
end
def create
#item = current_user.items.build(params[:item])
if #item.save
flash[:success] = "Your item has been saved"
redirect_to root_path
else
render 'new'
end
end
def destroy
#item.destroy
redirect_back_or root_path
end
end
and finally views/items/new.html.erb
<h1>Items Base</h1>
<div class="row">
<div class="span6 offset3">
<%= form_for(#item) do |f| %>
<%= f.label :title, "Title" %>
<%= f.text_field :title %>
<%= f.label :subtitle, "Subtitle" %>
<%= f.text_field :subtitle %>
<%= f.label :condition, "Condition" %>
<%= f.number_field :condition %>
<%= f.label :description, "Description" %>
<%= f.text_field :description %>
<%= f.label :day, "Day" %>
<%= f.text_field :day %>
<%= f.submit "List", class: "btn btn-large btn-primary" %>
<% end %>
</div>
</div>
Essentially I would like to be able to flash the message "Description too short!" when the user leaves it blank or below 20 characters, or flash the message "Title required" if it is left blank. Any thoughts on how best to achieve this. Also if anyone has any good resources on working with the flash it would be much appreciated. Thanks.

Answer 1

Here is how I do it:
Here is the form:
<%= form_for(#client) do |f| %>
<%= render 'shared/error_messages', object: f.object %>
<%= f.label :company_name %>
<%= f.text_field :company_name %>
<%= f.submit "Save changes", class: "btn btn-large btn-primary" %>
<% end %>
And here is my error_messages partial:
<% if object.errors.any? %>
<div id="error_explanation">
<div class="alert alert-error">
The form contains <%= pluralize(object.errors.count, "error") %>.
</div>
<ul>
<% object.errors.full_messages.each do |msg| %>
<li>* <%= msg %></li>
<% end %>
</ul>
</div>
<% end %>