No 'uninitialized constant' thrown when referencing AR class

'uninitialized constant' is not thrown for the following use case:

class Post < ActiveRecord::Base
  TEST_FOO = 123
  validates_presence_of :title, :message => TEST_FOO_WRONG # <- uninitialized constant!
  belongs_to :user
end

class User < ActiveRecord::Base
  has_many :posts
end

class PostsController < ApplicationController
  def index
    @post = User.first.posts.first # <- No uninitialized constant thrown!
    # @post = Post.first # <- Throws uninitialized constant
    render :text => @post.valid?
  end
end

Tested under Ruby 1.8.7, 1.9.2 and Rails 3.0.3