module ActiveSupport::Rescuable::ClassMethods

Registers exception classes with a handler to be called by rescue_with_handler.

rescue_from receives a series of exception classes or class names, and an exception handler specified by a trailing :with option containing the name of a method or a Proc object. Alternatively, a block can be given as the handler.

Handlers that take one argument will be called with the exception, so that the exception can be inspected when dealing with it.

Handlers are inherited. They are searched from right to left, from bottom to top, and up the hierarchy. The handler of the first class for which exception.is_a?(klass) holds true is the one invoked, if any.

class ApplicationController < ActionController::Base
  rescue_from User::NotAuthorized, with: :deny_access
  rescue_from ActiveRecord::RecordInvalid, with: :show_record_errors

  rescue_from "MyApp::BaseError" do |exception|
    redirect_to root_url, alert: exception.message
  end

  private
    def deny_access
      head :forbidden
    end

    def show_record_errors(exception)
      redirect_back_or_to root_url, alert: exception.record.errors.full_messages.to_sentence
    end
end

Exceptions raised inside exception handlers are not propagated up.

# File activesupport/lib/active_support/rescuable.rb, line 53
def rescue_from(*klasses, with: nil, &block)
  unless with
    if block_given?
      with = block
    else
      raise ArgumentError, "Need a handler. Pass the with: keyword argument or provide a block."
    end
  end

  klasses.each do |klass|
    key = if klass.is_a?(Module) && klass.respond_to?(:===)
      klass.name
    elsif klass.is_a?(String)
      klass
    else
      raise ArgumentError, "#{klass.inspect} must be an Exception class or a String referencing an Exception class"
    end

    # Put the new handler at the end because the list is read in reverse.
    self.rescue_handlers += [[key, with]]
  end
end

Matches an exception to a handler based on the exception class.

If no handler matches the exception, check for a handler matching the (optional) exception.cause. If no handler matches the exception or its cause, this returns nil, so you can deal with unhandled exceptions. Be sure to re-raise unhandled exceptions if this is what you expect.

begin
  # ...
rescue => exception
  rescue_with_handler(exception) || raise
end

Returns the exception if it was handled and nil if it was not.

# File activesupport/lib/active_support/rescuable.rb, line 90
def rescue_with_handler(exception, object: self, visited_exceptions: [])
  visited_exceptions << exception

  if handler = handler_for_rescue(exception, object: object)
    handler.call exception
    exception
  elsif exception
    if visited_exceptions.include?(exception.cause)
      nil
    else
      rescue_with_handler(exception.cause, object: object, visited_exceptions: visited_exceptions)
    end
  end
end