read_inheritable_attribute unexpectedly modified in production

Question

This is happening in Rails 3.0.7 and 3.0.9, WEBrick and Apache.

I have a module Reportable that has a method that writes an inheritable_attribute:

module Reportable
  module ClassMethods
    def add_report(report_name)
      instance_eval do
        write_inheritable_hash(:reportable_report_names,
          {report_name => {:dates => true, :details => 'something'})
        end
      end
    end
  end

  def self.included(base)
    base.extend(ClassMethods)
  end
end

Reportable is loaded in config/initializers and a class uses it:

class User < ActiveRecord::Base
  include Reportable
  add_report :report1
  add_report :report2
end

In production mode, the first time I load a page after the server starts, the attribute loads correctly:

User.read_inheritable_attribute(:reportable_report_names)
# => {:report1 => {:dates => true, :details => 'something'},
      :report2 => {:dates => true, :details => 'something'}}

But on the second page load:

User.read_inheritable_attribute(:reportable_report_names)
# => {:report1 => {:dates => true},
      :report2 => {:dates => true}}

It works as expected in development, and in the console in production mode. The problem only appears in a POST request on the web server in production mode. What gives?

A possible workaround would be making a class unloadable. Here you can find it explained: http://stackoverflow.com/questions/6853471/ruby-on-rails-unloadable — socjopata, Aug 10 '11 at 14:45

score 1 · Answer 1 · answered Aug 10 '11 at 14:45

1

Just tried, it works in all environments without problem. Any other details?

Otherwise:

why do you use instance_eval since you're working on the model itself?
you should use a splat to pass arguments

This would lead to:

module ClassMethods
  def add_reports(*report_names)
    report_names.each do |report_name|
      write_inheritable_hash(:loggable_report_names,
        {report_name => {:dates => true, :details => 'something'})
      end
    end
  end
end

And in model:

include Reportable
add_reports :report1, :report2

answered Aug 10 '11 at 14:45

apneadiving

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It's simplified... `Reportable` is included by many classes, each of which has a different set of reports. Each report is generated with a name (which is used to make a report method) and hash of options (which is stored in `:loggable_report_names` with the report method name as a key). There are also several other methods in the module to make retrieving the report names and their options easier. – jimworm Aug 10 '11 at 16:15
Not the answer... but an upvote for helping anyways. Thanks! I was expecting `read_inheritable_attribute` to return a value rather than a reference. – jimworm Aug 11 '11 at 09:36
Maybe you could create the objects in the 'included' block? – apneadiving Aug 11 '11 at 09:44
Yeah tried that too (and a gazillion other things... programming by permutation?), but it didn't work. The question didn't have enough info to be answered, I've put the eventual solution in an answer. – jimworm Aug 11 '11 at 10:16

jimworm · Accepted Answer · 2011-08-11T10:39:50.763

After a lot of headaches, the cause was that read_inheritable_attribute returned a reference rather than value.

The action throwing the error redirected to another action, which was the true cause of the problem. The module had another method:

def available_reports
  read_inheritable_attribute(:reportable_report_names)
end

And the other action did this:

# @model_reports = a hash of 'model_class_name'.to_sym => model.available_reports.dup
@model_reports.each do |model, model_reports|
  model_reports.each do |name, properties|
    properties = properties.keep_if... # remove per the view's requirements
  end
end

Because available_reports returned a reference, the dup did nothing and the keep_if deleted my precious values from the class variable.

The fix was:

properties = properties.dup.keep_if...

Thanks to those who took the time to check this question out even though there wasn't enough information in it to be answered properly.

read_inheritable_attribute unexpectedly modified in production

2 Answers2