Model classes are loaded before I18n is set when running tests.

Model classes should be always loaded after full framework configuration.

# config/locales/en.yml 
en:
  hello: "Hello world"

# config/locales/pl.yml 
pl:
  hello: "Witaj swiecie"

# app/models/my_model.rb
class MyModel < ActiveRecord::Base
  raise I18n.available_locales.inspect # just to demonstrate the bug
end

# test/unit/my_model_test.rb 
require 'test_helper'

class MyModelTest < ActiveSupport::TestCase
  test "the truth" { MyModel }
end

As you can see there are 2 files with 2 different locales and my class depends on them ex.:

class MyModel < ActiveRecord::Base
  I18n.available_locales.each do |locale|
    define_method("name_#{locale}") { ... do something ...} # This is what I actually do.
  end
end

When running from the console

> rails cosnole
Loading development environment (Rails 3.0.0)
ruby-1.9.2-head > MyModel
RuntimeError: [:en, :pl] 
    from /home/rupert/test/r3koma0/app/models/my_model.rb:3:in `<class:MyModel>'
    from /home/rupert/test/r3koma0/app/models/my_model.rb:1:in `<top (required)>'

everything is fine. Both locales ([:en, :pl]) are found and I'm happy.

However when running test:

ruby -Ilib:test test/unit/my_model_test.rb 

/home/rupert/test/r3koma0/app/models/my_model.rb:3:in `<class:MyModel>': [:en] (RuntimeError)
    from /home/rupert/test/r3koma0/app/models/my_model.rb:1:in `<top (required)>'
    from /home/rupert/.rvm/gems/ruby-1.9.2-head/gems/activesupport-3.0.0/lib/active_support/dependencies.rb:239:in `require'
    from /home/rupert/.rvm/gems/ruby-1.9.2-head/gems/activesupport-3.0.0/lib/active_support/dependencies.rb:239:in `block in require'
    from /home/rupert/.rvm/gems/ruby-1.9.2-head/gems/activesupport-3.0.0/lib/active_support/dependencies.rb:227:in `load_dependency'
    from /home/rupert/.rvm/gems/ruby-1.9.2-head/gems/activesupport-3.0.0/lib/active_support/dependencies.rb:239:in `require'
    from /home/rupert/.rvm/gems/ruby-1.9.2-head/gems/activesupport-3.0.0/lib/active_support/dependencies.rb:346:in `require_or_load'
    from /home/rupert/.rvm/gems/ruby-1.9.2-head/gems/activesupport-3.0.0/lib/active_support/dependencies.rb:300:in `depend_on'
    from /home/rupert/.rvm/gems/ruby-1.9.2-head/gems/activesupport-3.0.0/lib/active_support/dependencies.rb:216:in `require_dependency'
    from /home/rupert/.rvm/gems/ruby-1.9.2-head/gems/railties-3.0.0/lib/rails/engine.rb:138:in `block (2 levels) in eager_load!'
    from /home/rupert/.rvm/gems/ruby-1.9.2-head/gems/railties-3.0.0/lib/rails/engine.rb:137:in `each'
    from /home/rupert/.rvm/gems/ruby-1.9.2-head/gems/railties-3.0.0/lib/rails/engine.rb:137:in `block in eager_load!'
    from /home/rupert/.rvm/gems/ruby-1.9.2-head/gems/railties-3.0.0/lib/rails/engine.rb:135:in `each'
    from /home/rupert/.rvm/gems/ruby-1.9.2-head/gems/railties-3.0.0/lib/rails/engine.rb:135:in `eager_load!'
    from /home/rupert/.rvm/gems/ruby-1.9.2-head/gems/railties-3.0.0/lib/rails/application.rb:108:in `eager_load!'
    from /home/rupert/.rvm/gems/ruby-1.9.2-head/gems/railties-3.0.0/lib/rails/application/finisher.rb:41:in `block in <module:Finisher>'
    from /home/rupert/.rvm/gems/ruby-1.9.2-head/gems/railties-3.0.0/lib/rails/initializable.rb:25:in `instance_exec'
    from /home/rupert/.rvm/gems/ruby-1.9.2-head/gems/railties-3.0.0/lib/rails/initializable.rb:25:in `run'
    from /home/rupert/.rvm/gems/ruby-1.9.2-head/gems/railties-3.0.0/lib/rails/initializable.rb:50:in `block in run_initializers'
    from /home/rupert/.rvm/gems/ruby-1.9.2-head/gems/railties-3.0.0/lib/rails/initializable.rb:49:in `each'
    from /home/rupert/.rvm/gems/ruby-1.9.2-head/gems/railties-3.0.0/lib/rails/initializable.rb:49:in `run_initializers'
    from /home/rupert/.rvm/gems/ruby-1.9.2-head/gems/railties-3.0.0/lib/rails/application.rb:134:in `initialize!'
    from /home/rupert/.rvm/gems/ruby-1.9.2-head/gems/railties-3.0.0/lib/rails/application.rb:77:in `method_missing'
    from /home/rupert/test/r3koma0/config/environment.rb:5:in `<top (required)>'
    from <internal:lib/rubygems/custom_require>:29:in `require'
    from <internal:lib/rubygems/custom_require>:29:in `require'
    from /home/rupert/test/r3koma0/test/test_helper.rb:2:in `<top (required)>'
    from <internal:lib/rubygems/custom_require>:29:in `require'
    from <internal:lib/rubygems/custom_require>:29:in `require'
    from test/unit/my_model_test.rb:1:in `<main>

As you can see during the test something is loaded in a different order and I18n is not set yet, thus only one locale ([:en]) is found. Maybe eager_load! is called to early before loading everything that is related to I18n ?

Changing the files to :

# test/unit/my_model_test.rb 
require 'test_helper'
class MyModelTest < ActiveSupport::TestCase
  test "the truth" {raise I18n.available_locales.inspect}
end

# app/models/my_model.rb 
class MyModel < ActiveRecord::Base
end

shows that after everything is loaded in test environment, locales are again properly returned:

ruby -Ilib:test test/unit/my_model_test.rb 
Loaded suite test/unit/my_model_test
Started
E
Finished in 0.018779 seconds.

  1) Error:
test_the_truth(MyModelTest):
RuntimeError: [:en, :pl]
    test/unit/my_model_test.rb:6:in `block in <class:MyModelTest>'