The DOM id convention is to use the singular form of an object or class with the id following an underscore. If no id is found, prefix with "new_" instead. Examples:

dom_id(Post.new(:id => 45)) # => "post_45"
dom_id(Post.new)            # => "new_post"

If you need to address multiple instances of the same class in the same view, you can prefix the dom_id:

dom_id(Post.new(:id => 45), :edit) # => "edit_post_45"

Source Code

# File action_controller/record_identifier.rb, line 65
def dom_id(record, prefix = nil) 
  prefix ||= 'new' unless record.id
  [ prefix, singular_class_name(record), record.id ].compact * '_'
end