rails3: ActiveRecord::AutosaveAssociation.validate_single_association being triggered when it shouldn't

I'm noticing Rails call ActiveRecord::AutosaveAssociation.validate_single_association when I save a model that belongs to another, though it shouldn't be according to the documentation. The documentation says that this method will be called when :validate or :autosave is turned on for an association, and I haven't done that.

This is causing me an issue because of the way I have set up my models and the actions that are performed before validation occurs. Let's take for example a User and Post model.

• A user has many posts.
• A post belongs to a user.

abstract_model.rb

class AbstractModel < ActiveRecord::Base
  self.abstract_class = true
  before_validation :foo
  attr_accessor :special_msg

  def foo
    puts special_msg
  end
end

user.rb

class User < AbstractModel
   has_many :posts
end

post.rb

class Post < AbstractModel
  belongs_to :user
end

Notice that I haven't defined :validate or :autosave on the association.

PostsController.update

def update
  @post = Post.find(params[:id]
  @post.special_msg = "Dumbledore dies, so does Hedwig"
  @post.update_attributes(params[:post])
end

Using the debugger, I noticed the following:

• AbstractModel.foo gets called 3 times from the last line of PostsController.update
  • The first and second calls:
    • It is being called on an instance of Post
    • It is being called via
      • ActiveRecord::Validations
      • ActiveModel::Validations
    • special_msg is not blank
  • The third call:
    • It is being called on a instance of User, not Post
    • it is being called via
      • ActiveRecord::Validations
      • ActiveRecord::AutosaveAssociation.validate_single_association
      • ActiveModel::Validations
    • special_msg is blank (nil)

I've actually posted this question on Stackoverflow as well, but so far, no answers.