module ActiveRecord::AutosaveAssociation

def save_collection_association(reflection)

ActiveRecord::Base after the AutosaveAssociation module, which it does by default.
This all happens inside a transaction, _if_ the Transactions module is included into

with mark_for_destruction.
In addition, it destroys all children that were marked for destruction

:autosave is enabled on the association.
Saves any new associated records, or all loaded autosave associations if 
def save_collection_association(reflection)
  if association = association_instance_get(reflection.name)
    autosave = reflection.options[:autosave]
    if records = associated_records_to_validate_or_save(association, @new_record_before_save, autosave)
      records.each do |record|
        next if record.destroyed?
        if autosave && record.marked_for_destruction?
          association.destroy(record)
        elsif autosave != false && (@new_record_before_save || record.new_record?)
          if autosave
            saved = association.send(:insert_record, record, false, false)
          else
            association.send(:insert_record, record)
          end
        elsif autosave
          saved = record.save(:validate => false)
        end
        raise ActiveRecord::Rollback if saved == false
      end
    end
    # reconstruct the SQL queries now that we know the owner's id
    association.send(:construct_sql) if association.respond_to?(:construct_sql)
  end
end