Welcome to the lesson on error handling in Ruby on Rails applications. In previous lessons, we integrated a database, added a column to the todos table, and configured middleware to enhance our application's security. Now, we’ll focus on handling errors gracefully to ensure our application provides meaningful feedback to users when things go wrong.

Error handling is crucial in enterprise applications. It helps maintain a smooth user experience by properly managing and displaying errors. By the end of this lesson, you will learn to implement a custom exception handler that catches and manages common errors in your Rails application. Let's get started!

Ruby on Rails provides built-in mechanisms to handle exceptions gracefully. One powerful tool is the rescue_from method, which allows us to specify how to handle specific exceptions in our controller.

Here’s a simple example of how rescue_from works:

Ruby
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1class TodosController < ApplicationController
2  rescue_from ActiveRecord::RecordNotFound, with: :record_not_found
3
4  def show
5    @todo = Todo.find(params[:id])
6    render json: @todo
7  end
8
9  private
10
11  def record_not_found(exception)
12    render json: { error: exception.message }, status: :not_found
13  end
14end

In this code, when an ActiveRecord::RecordNotFound exception is raised, the record_not_found method is called, which renders a JSON response with an error message and a 404 Not Found status.

To handle exceptions more elegantly across multiple controllers, we can create a Concern for exception handling. Concerns use ActiveSupport::Concern, which simplifies the inclusion of shared behavior in Rails controllers:

Ruby
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1module ExceptionHandler
2  extend ActiveSupport::Concern
3
4  included do
5    rescue_from ActiveRecord::RecordNotFound do |e|
6      render json: { error: e.message }, status: :not_found
7    end
8
9    rescue_from ActiveRecord::RecordInvalid, with: :unprocessable_entity_response
10  end
11
12  private
13
14  def unprocessable_entity_response(exception)
15    render json: { error: exception.message }, status: :unprocessable_entity
16  end
17end

This ExceptionHandler module uses rescue_from to handle two common exceptions: ActiveRecord::RecordNotFound and ActiveRecord::RecordInvalid. The unprocessable_entity_response method provides customized error responses for invalid records.

• ActiveSupport::Concern: This module is used to create reusable concerns in Rails. It provides methods for including functionality in a modular way.
• rescue_from: This method specifies how to handle exceptions in controllers. In our example, it handles ActiveRecord::RecordNotFound by rendering a JSON error message with a 404 status. It also handles ActiveRecord::RecordInvalid using the unprocessable_entity_response method.

To use our custom exception handler, we include the ExceptionHandler module in our controllers. This ensures that the exception handling behavior is shared across controllers:

Ruby
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1class ApplicationController < ActionController::API
2  include ExceptionHandler
3end

By including ExceptionHandler in ApplicationController, all controllers that inherit from ApplicationController will have access to the exception handling methods.

Handling RecordNotFound and RecordInvalid errors improves user feedback and helps diagnose issues effectively:

• RecordNotFound: This exception occurs when trying to find a record that doesn't exist.
• RecordInvalid: This exception occurs when trying to save a record that fails validation.

Here's an example of how the show and create actions handle these exceptions:

Ruby
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1class TodosController < ApplicationController
2  def show
3    @todo = Todo.find(params[:id])
4    render json: @todo
5  end
6
7  def create
8    @todo = Todo.create!(todo_params)
9    render json: @todo, status: :created
10  end
11
12  private
13
14  def todo_params
15    params.require(:todo).permit(:title, :description, :file)
16  end
17end

In the show action, if the Todo record is not found, the record_not_found method in our ExceptionHandler module will handle the exception. Similarly, in the create action, if the Todo record fails validation, the unprocessable_entity_response method will handle the exception.

In this lesson, we learned how to enhance the robustness of our Ruby on Rails application by implementing custom error handling. We created an ExceptionHandler module to manage common exceptions and included it in our controllers. This allows our application to handle errors gracefully and provide meaningful feedback to users.

Key points covered:

• The importance of error handling in enterprise applications.
• Using rescue_from to manage exceptions in controllers.
• Creating a custom ExceptionHandler module to share error handling logic.
• Including the ExceptionHandler module in controllers to handle common exceptions.

Next, you’ll get hands-on practice implementing and testing the error handling concepts discussed in this lesson. Remember, a robust error handling mechanism is essential for providing a seamless user experience and debugging issues efficiently.

Great job reaching this point! Let’s move on to the practice exercises to solidify what you’ve learned.