validates :presence vs validates_presence_of using rails 3

I have a couple of simple models that are associated like so:

MODELS

class Task < ActiveRecord::Base
  belongs_to :user
  validates :name, :presence => true, :message => 'Name cannot be blank, Task not saved'
end

class User < ActiveRecord::Base
  has_many :tasks
end

VIEW has a call in it like so: user.tasks <-- then I loop through the tasks

The Issue:

In the task model --

when I use:

validates :name, :presence => true ,  :message => 'Name cannot be blank, Task not saved'

I get a 500 error:

ActionView::Template::Error (uninitialized constant User::Task):
NameError in View file

when I use:

validates_presence_of :name

Everything works.

I thought the both validates methods above where the same...is the issue have to do with associations and how validation tie into associated models. I have a hunch that something is going on with the way things are associated, but it is just a hunch.

Any help will be appreciated. Thank very much.