How to Raise and Rescue Errors in Rails

Raising and rescuing errors in Rails is relatively easy. All you need to do is make a custom error class, raise it (usually in a model) and rescue it (usually in a controller).

Start by making a custom error class that inherits from StandardError. This will give you a more descriptive error instead of raising the generic StandardError, so you can better deduce where the error came from. I will use a hypothetical example of an error that is raised when a User is being created.

class UserCreateError < StandardError
end

Each custom error class should have it’s own file which you can put in an app/errors directory.

Here I will raise the error in a User model method.

class User < ActiveRecord::Base
  def self.example_method
    raise UserCreateError
  end
end

When the create action is called on the controller, it will call User.example_method, which will raise the error. The error then goes back up to the controller where it is rescued.

class UsersController < ApplicationController
  def create
    User.example_method
  rescue UserCreateError
    render json: "Invalid User Information", status: :bad_request
  end
end

Note the indentation of the rescue is on the same column as the def.

Whatever you put underneath the rescue will be run if there is an error.

Here I render a custom json error message and specify an optional status code.

And that’s pretty much all there is too it.