Rails 2.3 ActiveSupport::Concern error

Question

I am trying to get an already developed app running built on Rails 2.3.11, and I am getting a weird error:

vendor/rails/activesupport/lib/active_support/dependencies.rb:466:in `load_missing_constant': uninitialized constant ActiveSupport::Concern (NameError)

As I understand ActiveSupport::Concern was not available before Rails 3. What is really wrong?

My gem list :

*** LOCAL GEMS ***

* abstract (1.0.0)
actionmailer (2.3.11, 2.1.2)
actionpack (2.3.11, 2.1.2)
activerecord (2.3.11, 2.1.2)
activeresource (2.3.11, 2.1.2)
activesupport (3.2.2, 2.3.11)
arel (2.2.1)
breadcrumbs_on_rails (2.2.0, 1.0.1)
builder (3.0.0)
bundler (1.1.0)
ckeditor (3.4.3)
erubis (2.7.0)
GData (0.0.4)
hike (1.2.1)
hoe (2.16.0)
hpricot (0.8.4)
htmlentities (4.2.4)
i18n (0.6.0)
journey (1.0.3)
json (1.6.5)
libxml-ruby (2.2.2, 1.1.4)
mime-types (1.17.2)
multi_json (1.1.0)
narray (0.6.0.1)
nokogiri (1.4.4)
pg (0.13.2, 0.8.0)
postgres (0.8.1)
rack (1.4.1, 1.2.1, 1.1.3)
rack-cache (1.2)
rack-ssl (1.3.2)
rack-test (0.6.1)
rails (2.3.11, 2.1.2)
rake (0.8.7)
rdoc (3.12)
roo (1.3.11)
ruby-ole (1.2.11.3)
ruby-units (1.1.5, 1.1.3)
rubygems-update (1.6.2, 1.4.0, 1.3.7, 1.3.6)
rubyzip (0.9.6.1)
spreadsheet (0.6.8)
sprockets (2.1.2)
tilt (1.3.3)
will_paginate (2.3.16)

Answer 1

It looks to me like you have ActiveSupport 3.2.2 installed along with 2.3.x. I would try removing that version of the gem - could be something's trying to load ActiveSupport 3.2.2, then AS is barfing because it has version confusion?

Uninstall a version of a gem with:

gem uninstall -v 3.2.2 activesupport

(I would suggest using RVM gemsets to keep gem versions away from each other - for example, create a gemset that you just use for this project).