person_spec.rb

person.rb

class Person < ActiveRecord::Base
  
  validates :first_name, :presence => true 
  validates :last_name, :presence => true
  
end

person_spec.rb

require 'spec_helper'

describe Person do
  
  # Yes, yes, this could be build from a
  # Factory...
  before(:each) do
    @valid_attributes = {
      :first_name => "Joe",
      :last_name => "Bloggs"
    }
  end
  
  it "creates a valid Person given valid attributes" do
    lambda {
      Person.create(@valid_attributes)
# Alternate method...
#    }.should change(Person, :count).from(0).to(1)
    }.should change(Person, :count).by(1)
  end
  
# Syntax examples. 
  it "should save the first name" do
    p = Person.create({:first_name => "John", :last_name => "Smith"})
    p.first_name.should == "John"
  end
  
  it "should save the last name" do
    p = Person.create({:first_name => "John", :last_name => "Smith"})
    p.last_name.should == "Smith"
  end
  
# The following two are dangerous. If only the 
# invalid part is checked, the test produces a 
# false positive. A better way to do this would be 
# use @valid_attributes, then override the attribute
# being tested. Try it yourself!
  it "must have a first name" do
    lambda {
      #Person.create({:first_name => ''})    
      Person.create({:first_name => '', :last_name => "Smith"})    
    }.should_not change(Person, :count)
  end

  it "must have a last name" do
    lambda {
      #Person.create({:last_name => ''})    
      Person.create({:first_name => "John", :last_name => ''})    
    }.should_not change(Person, :count)
  end
  
# Same issue as above, try it yourself.
  it "must have a first name" do
    #p = Person.new({:first_name => ''}) # creates a false positive
    p = Person.new({:first_name => '', :last_name => "Smith"})
    p.should_not be_valid
  end

  it "must have a last name" do
    p = Person.new({:first_name => 'John', :last_name => ''})
    p.should_not be_valid
  end

end

Reference validation tests from Wolf Arnold's Efficient Test Driven Rails Development class summer 2010.