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RecordIdentifier encapsulates methods used by various ActionView helpers to associate records with DOM elements.

Consider for example the following code that form of post:

<%= form_for(post) do |f| %>
  <%= f.text_field :body %>
<% end %>

When post is a new, unsaved ActiveRecord::Base instance, the resulting HTML is:

<form class="new_post" id="new_post" action="/posts" accept-charset="UTF-8" method="post">
  <input type="text" name="post[body]" id="post_body" />

When post is a persisted ActiveRecord::Base instance, the resulting HTML is:

<form class="edit_post" id="edit_post_42" action="/posts/42" accept-charset="UTF-8" method="post">
  <input type="text" value="What a wonderful world!" name="post[body]" id="post_body" />

In both cases, the id and class of the wrapping DOM element are automatically generated, following naming conventions encapsulated by the RecordIdentifier methods dom_id and dom_class:

dom_id(Post.new)         # => "new_post"
dom_class(Post.new)      # => "post"
dom_id(Post.find 42)     # => "post_42"
dom_class(Post.find 42)  # => "post"

Note that these methods do not strictly require Post to be a subclass of ActiveRecord::Base. Any Post class will work as long as its instances respond to to_key and model_name, given that model_name responds to param_key. For instance:

class Post
  attr_accessor :to_key

  def model_name
    OpenStruct.new param_key: 'post'

  def self.find(id)
    new.tap { |post| post.to_key = [id] }
JOIN = "_".freeze
NEW = "new".freeze
Instance Public methods
dom_class(record_or_class, prefix = nil)

The DOM class convention is to use the singular form of an object or class.

dom_class(post)   # => "post"
dom_class(Person) # => "person"

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

dom_class(post, :edit)   # => "edit_post"
dom_class(Person, :edit) # => "edit_person"
# File actionview/lib/action_view/record_identifier.rb, line 73
def dom_class(record_or_class, prefix = nil)
  singular = model_name_from_record_or_class(record_or_class).param_key
  prefix ? "#{prefix}#{JOIN}#{singular}" : singular
dom_id(record, prefix = nil)

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.

dom_id(Post.find(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.find(45), :edit) # => "edit_post_45"
dom_id(Post.new, :custom)    # => "custom_post"
# File actionview/lib/action_view/record_identifier.rb, line 88
def dom_id(record, prefix = nil)
  if record_id = record_key_for_dom_id(record)
    "#{dom_class(record, prefix)}#{JOIN}#{record_id}"
    dom_class(record, prefix || NEW)
Instance Private methods

Returns a string representation of the key attribute(s) that is suitable for use in an HTML DOM id. This can be overwritten to customize the default generated string representation if desired. If you need to read back a key from a #dom_id in order to query for the underlying database record, you should write a helper like 'person_record_from_dom_id' that will extract the key either based on the default implementation (which just joins all key attributes with '_') or on your own overwritten version of the method. By default, this implementation passes the key string through a method that replaces all characters that are invalid inside DOM ids, with valid ones. You need to make sure yourself that your dom ids are valid, in case you overwrite this method.

# File actionview/lib/action_view/record_identifier.rb, line 106
def record_key_for_dom_id(record) # :doc:
  key = convert_to_model(record).to_key
  key ? key.join(JOIN) : key