In case you want directly add a setting by insert through the model you get an error

insert the following test case into setting_test.rb

def test_insert_name_not_in_settings_yml
setting = Setting.new(:name => "does_not_exist", :value => "should_not_be_allowed" )
assert !setting.save
end

Then you get the errors below:

test_insert_name_not_in_settings_yml(SettingTest):
NoMethodError: undefined method `[]' for nil:NilClass
    /redmine/app/models/setting.rb:86:in `block in '
    /usr/lib64/ruby/gems/1.9.1/gems/activesupport-3.2.15/lib/active_support/callbacks.rb:425:in `_run__2686202268941026958__validate__1184212570148052443__callbacks'
    /usr/lib64/ruby/gems/1.9.1/gems/activesupport-3.2.15/lib/active_support/callbacks.rb:405:in `__run_callback'
    /usr/lib64/ruby/gems/1.9.1/gems/activesupport-3.2.15/lib/active_support/callbacks.rb:385:in `_run_validate_callbacks'
    /usr/lib64/ruby/gems/1.9.1/gems/activesupport-3.2.15/lib/active_support/callbacks.rb:81:in `run_callbacks'
    /usr/lib64/ruby/gems/1.9.1/gems/activemodel-3.2.15/lib/active_model/validations.rb:228:in `run_validations!'
    /usr/lib64/ruby/gems/1.9.1/gems/activemodel-3.2.15/lib/active_model/validations/callbacks.rb:53:in `block in run_validations!'
    /usr/lib64/ruby/gems/1.9.1/gems/activesupport-3.2.15/lib/active_support/callbacks.rb:403:in `_run__2686202268941026958__validation__1184212570148052443__callbacks'
    /usr/lib64/ruby/gems/1.9.1/gems/activesupport-3.2.15/lib/active_support/callbacks.rb:405:in `__run_callback'
    /usr/lib64/ruby/gems/1.9.1/gems/activesupport-3.2.15/lib/active_support/callbacks.rb:385:in `_run_validation_callbacks'
    /usr/lib64/ruby/gems/1.9.1/gems/activesupport-3.2.15/lib/active_support/callbacks.rb:81:in `run_callbacks'
    /usr/lib64/ruby/gems/1.9.1/gems/activemodel-3.2.15/lib/active_model/validations/callbacks.rb:53:in `run_validations!'
    /usr/lib64/ruby/gems/1.9.1/gems/activemodel-3.2.15/lib/active_model/validations.rb:195:in `valid?'
    /usr/lib64/ruby/gems/1.9.1/gems/activerecord-3.2.15/lib/active_record/validations.rb:69:in `valid?'
    /usr/lib64/ruby/gems/1.9.1/gems/activerecord-3.2.15/lib/active_record/validations.rb:77:in `perform_validations'
    /usr/lib64/ruby/gems/1.9.1/gems/activerecord-3.2.15/lib/active_record/validations.rb:50:in `save'
    /usr/lib64/ruby/gems/1.9.1/gems/activerecord-3.2.15/lib/active_record/attribute_methods/dirty.rb:22:in `save'
    /usr/lib64/ruby/gems/1.9.1/gems/activerecord-3.2.15/lib/active_record/transactions.rb:259:in `block (2 levels) in save'
    /usr/lib64/ruby/gems/1.9.1/gems/activerecord-3.2.15/lib/active_record/transactions.rb:313:in `block in with_transaction_returning_status'
    /usr/lib64/ruby/gems/1.9.1/gems/activerecord-3.2.15/lib/active_record/connection_adapters/abstract/database_statements.rb:192:in `transaction'
    /usr/lib64/ruby/gems/1.9.1/gems/activerecord-3.2.15/lib/active_record/transactions.rb:208:in `transaction'
    /usr/lib64/ruby/gems/1.9.1/gems/activerecord-3.2.15/lib/active_record/transactions.rb:311:in `with_transaction_returning_status'
    /usr/lib64/ruby/gems/1.9.1/gems/activerecord-3.2.15/lib/active_record/transactions.rb:259:in `block in save'
    /usr/lib64/ruby/gems/1.9.1/gems/activerecord-3.2.15/lib/active_record/transactions.rb:270:in `rollback_active_record_state!'
    /usr/lib64/ruby/gems/1.9.1/gems/activerecord-3.2.15/lib/active_record/transactions.rb:258:in `save'
    test/unit/setting_test.rb:93:in `test_insert_name_not_in_settings_yml'
    /usr/lib64/ruby/gems/1.9.1/gems/mocha-0.14.0/lib/mocha/integration/mini_test/version_230_to_2101.rb:36:in `run'

Same if you call the following on console

s = Setting.new(:name => "does_not_exist", :value => "should_not_be_allowed" )
s.valid?

The reason is in the app/model/setting.rb

validates_numericality_of :value, :only_integer => true, :if => Proc.new { |setting| @@available_settings[setting.name]['format'] == 'int' }

The Proc breaks if name (the key) is not in the hash.

Fix for it is to check if key is in hash inside the proc before using the non existing key so that check of numericality is skipped when key does not exist:

validates_numericality_of :value, :only_integer => true, :if => Proc.new { |setting| @@available_settings.has_key?(setting) ? @@available_settings[setting.name]['format'] == 'int' : false}