Rails core - Feb 2013 - Inconsistent validation behavior for has_many :through association

The API documentation for ActiveRecord associations here: 
http://api.rubyonrails.org/classes/ActiveRecord/Associations/ClassMethods.html, 
says:

   - Adding an object to a collection (has_many or has_and_belongs_to_many) 
   automatically saves that object, except if the parent object (the owner of 
   the collection) is not yet stored in the database.
   - If saving any of the objects being added to a collection (via push or 
   similar) fails, then push returns false
   
Similarly in the Rails Guides - 
http://guides.rubyonrails.org/association_basics.html#has_many-association-reference
 :

-------------
4.3.3 When are Objects Saved?
When you assign an object to a has_many association, that object is 
automatically saved (in order to update its foreign key). If you assign 
multiple objects in one statement, then they are all saved.

If any of these saves fails due to validation errors, then the assignment 
statement returns false and the assignment itself is cancelled.
-------------


This all seems to work fine for regular has_many associations, but for 
has_many :through associations if I add an invalid record to an existing 
parent via << I get an ActiveRecord::RecordInvalid exception rather than 
false. Looking at the code, it is evident why...


associations/collection_proxy.rb
======================class ActiveRecord::Associations:CollectionProxy
      def <<(*records)
        proxy_association.concat(records) && self
      end
      alias_method :push, :<<
end


associations/has_many_through_association.rb
=================================class
ActiveRecord::Associations::HasManyThroughAssociation <
HasManyAssociation
      def concat(*records)
        unless owner.new_record?
          records.flatten.each do |record|
            raise_on_type_mismatch(record)
            record.save! if record.new_record?
          end
        end
        super
      end
end

Since the record is saved with save!, an invalid record raises the 
exception.

Is this unintended, or is the documentation misleading?

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