Getting a list of existing Rails Error classes for re-use/inheritance

Question

Often I need to throw a custom(ized) error. Like when a resource cannot be found due to a mismatch in parameters or so.

I prefer to throw existing errors or, throw an error that is inherited from an existing error. That way, I don't introduce Error classes that were already defined and could have been used perfectly (DRY). But it also allows to keep wording and style the same, by inheriting and simply changing a word or two to clarify the difference with the original Error.

For example:

Foo.new
Foo.some_external_id = nil
Foo.fetch_external_resource
# => InvalidOptions: Calling Foo#fetch_external_resource with nil is invalid

I am quite sure such errors are already defined. In fact, after reading trough many lines of code, I found my MongoID driver has Mongoid::Errors::InvalidOptions: Calling Document#find with nil is invalid.

Is there a list of available Error classes in Ruby Core and Ruby on Rails? Is there a way to get such a list for your current project?

Is it smart at all to re-use and/or inherit existing errors, or should I maintain my own, custom set instead?

Answer 1

While this list may change, and it's best to still use Ryan's solution, the list for Rails 4 (limited to Rails classes and not other gems):

AbstractController::ActionNotFound
AbstractController::DoubleRenderError
AbstractController::Error
AbstractController::Helpers::ClassMethods::MissingHelperError
ActionController::ActionControllerError
ActionController::BadRequest
ActionController::InvalidAuthenticityToken
ActionController::MethodNotAllowed
ActionController::MissingFile
ActionController::NotImplemented
ActionController::ParameterMissing
ActionController::RedirectBackError
ActionController::RenderError
ActionController::RoutingError
ActionController::SessionOverflowError
ActionController::UnknownController
ActionController::UnknownFormat
ActionController::UnknownHttpMethod
ActionController::UnpermittedParameters
ActionController::UrlGenerationError
ActionDispatch::Cookies::CookieOverflow
ActionDispatch::IllegalStateError
ActionDispatch::Journey::Router::RoutingError
ActionDispatch::ParamsParser::ParseError
ActionDispatch::RemoteIp::IpSpoofAttackError
ActionDispatch::Session::SessionRestoreError
ActionView::Helpers::NumberHelper::InvalidNumberError
ActiveModel::ForbiddenAttributesError
ActiveModel::MissingAttributeError
ActiveRecord::ActiveRecordError
ActiveRecord::AdapterNotFound
ActiveRecord::AdapterNotSpecified
ActiveRecord::AssociationTypeMismatch
ActiveRecord::AttributeAssignmentError
ActiveRecord::ConfigurationError
ActiveRecord::ConnectionNotEstablished
ActiveRecord::ConnectionTimeoutError
ActiveRecord::DangerousAttributeError
ActiveRecord::DeleteRestrictionError
ActiveRecord::DuplicateMigrationNameError
ActiveRecord::DuplicateMigrationVersionError
ActiveRecord::EagerLoadPolymorphicError
ActiveRecord::HasAndBelongsToManyAssociationForeignKeyNeeded
ActiveRecord::HasManyThroughAssociationNotFoundError
ActiveRecord::HasManyThroughAssociationPointlessSourceTypeError
ActiveRecord::HasManyThroughAssociationPolymorphicSourceError
ActiveRecord::HasManyThroughAssociationPolymorphicThroughError
ActiveRecord::HasManyThroughCantAssociateNewRecords
ActiveRecord::HasManyThroughCantAssociateThroughHasOneOrManyReflection
ActiveRecord::HasManyThroughCantDissociateNewRecords
ActiveRecord::HasManyThroughNestedAssociationsAreReadonly
ActiveRecord::HasManyThroughSourceAssociationNotFoundError
ActiveRecord::HasOneThroughCantAssociateThroughCollection
ActiveRecord::IllegalMigrationNameError
ActiveRecord::ImmutableRelation
ActiveRecord::InvalidForeignKey
ActiveRecord::InverseOfAssociationNotFoundError
ActiveRecord::IrreversibleMigration
ActiveRecord::MultiparameterAssignmentErrors
ActiveRecord::NestedAttributes::TooManyRecords
ActiveRecord::PendingMigrationError
ActiveRecord::PreparedStatementInvalid
ActiveRecord::ReadOnlyAssociation
ActiveRecord::ReadOnlyRecord
ActiveRecord::RecordInvalid
ActiveRecord::RecordNotDestroyed
ActiveRecord::RecordNotFound
ActiveRecord::RecordNotSaved
ActiveRecord::RecordNotUnique
ActiveRecord::Rollback
ActiveRecord::SerializationTypeMismatch
ActiveRecord::StaleObjectError
ActiveRecord::StatementInvalid
ActiveRecord::SubclassNotFound
ActiveRecord::Tasks::DatabaseAlreadyExists
ActiveRecord::Tasks::DatabaseNotSupported
ActiveRecord::ThrowResult
ActiveRecord::TransactionIsolationError
ActiveRecord::Transactions::TransactionError
ActiveRecord::UnknownAttributeError
ActiveRecord::UnknownMigrationVersionError
ActiveRecord::UnknownPrimaryKey
ActiveRecord::WrappedDatabaseException
ActiveSupport::JSON::Encoding::CircularReferenceError
ActiveSupport::MessageVerifier::InvalidSignature
ActiveSupport::SafeBuffer::SafeConcatError
ActiveSupport::XMLConverter::DisallowedType

Answer 2

There's a mostly adequate solution here: http://www.ruby-forum.com/topic/158088

Since this question is unanswered, yet coming up at the top of Google search results, I decided to wrap Frederick Cheung's solution up in a rake task and post it here.

Drop the following in lib/tasks/exceptions.rake

namespace :exceptions do
  task :list => :environment do
    exceptions = []

    ObjectSpace.each_object(Class) do |k|
      exceptions << k if k.ancestors.include?(Exception)
    end

    puts exceptions.sort { |a,b| a.to_s <=> b.to_s }.join("\n")
  end
end

Run it with:

bundle exec rake exceptions:list

If you're still on Rails 2, or not using Bundler, leave off the bundle exec

This list is probably adequate, but not exhaustive. For example, ActiveResource defines several exceptions such as ActiveResource::ConnectionError and ActiveResource::TimeoutError that are not appearing when I run this task. Maybe someone else can enlighten me as to why.

Answer 3

A shorter option is available in Rails thanks to ActiveSupport:

puts Exception.descendants.sort_by(&:name)

You can also check out the source to see how they handle it.