Ruby On Rails Exceptions

Understand Ruby on Rails exceptions, including example of NameError: undefined local variable or method.

Ruby On Rails Exceptions

Exceptions in Ruby on Rails are used to manage errors, and can be used to supply helpful information about what went wrong. An exception is an object of the class Exception or a subclass of it. When an exception is raised, the current process is stopped and control is transferred to the exception handler.

When an exception is raised, it can be handled by a rescue block. The general form of a rescue block looks like this:

begin
  # code that may raise an exception
rescue
  # code that handles the exception
end

The code inside the begin block is executed, and if an exception is raised, control is transferred to the rescue block. This block performs whatever is necessary to handle the exception. When the handling is complete, execution continues with the statement following the end.

The rescue block can contain code to handle specific types of exceptions. This is done by providing one or more classes in the rescue clause. For example:

begin
  # code that may raise an exception
rescue StandardError
  # code that handles the StandardError exception
end

In this example, the rescue block will only be executed if a StandardError (or a subclass of it) is raised. If any other type of exception is raised, it will be propagated to the caller.

If you want to access information about the exception that was raised, you can do so by assigning the exception object to a variable. This is done by providing the variable name after the rescue clause. For example:

begin
  # code that may raise an exception
rescue StandardError => e
  # code that handles the StandardError exception
  # e is the exception object
end

In this example, the exception object is assigned to the variable e, which can be used to access information about the exception. For example, the message associated with the exception can be accessed with e.message.