people_controller.rb

class PeopleController < ApplicationController
  def create
    begin
      p = Person.new params[:person]
      p.save
      render :text => "yay!"
    rescue
      render :text => "oops!"
    end
  end
end

people_controller_spec.rb

require File.expand_path(File.dirname(__FILE__) + '/../spec_helper')

describe PeopleController do
  integrate_views

  it "should show a message when there's an error'" do
    post :create, :person => {:name => nil}
    response.body.should == "oops!"
  end
end

people_migration.rb

class CreatePeople < ActiveRecord::Migration
  def self.up
    create_table :people do |t|
      t.string :name, :null => false

      t.timestamps
    end
  end

  def self.down
    drop_table :people
  end
end