Question

Here is an example from a failed spec:

2) widgets/new renders new sentiment form
     Failure/Error: render
     ActionView::Template::Error:
       undefined method `each' for nil:NilClass
     # ./app/views/widgets/new.html.erb:13:in `_app_views_widgets_new_html_erb__3591031757452814763_2164570860'
     # /Users/user/.rvm/gems/ruby-1.9.3-p194@myapp/gems/actionpack-3.2.7/lib/action_view/template.rb:145:in `block in render'
     # /Users/user/.rvm/gems/ruby-1.9.3-p194@myapp/gems/activesupport-3.2.7/lib/active_support/notifications.rb:123:in `block in instrument'
     .......

What does all this mean: _app_views_widgets_new_html_erb__3591031757452814763_2164570860? Is it rendering a temporary file somewhere? Why these particular numbers?

Answer 1

When rails renders your templates, it first compiles them to real ruby code. You can see this in the rails source in actionpack/lib/action_view/template.rb.

This is done 3 steps.

1. It creates a string that contains ruby code like @output_buffer += "some_text"; @output_buffer += some_expression; ... for your erb (or haml, or whatever you prefer) template.
2. It defines a method using eval.
3. When the method is run the first time, the ruby VM automatically compiles it to bytecode.

The name of the method containing the compiled code is created using the scheme below (see line 325 of template.rb):

def method_name #:nodoc:
  @method_name ||= "_#{identifier_method_name}__#{@identifier.hash}_#{__id__}".gsub('-', "_")
end

So the numbers are actually the hash of the @identifier (which is usally the path of the template file) and the object id of the current template instance.