Before and after filters in Ruby on Rails - Step-by-Step Execution

Concept Flow - Before and after filters

Request received

↓

Run before filters

↓

Run controller action

↓

Run after filters

↓

Send response

Rails runs before filters first, then the action, then after filters before sending the response.

Execution Sample

Ruby on Rails

class PostsController < ApplicationController
  before_action :authenticate_user
  after_action :log_action

  def show
    # show post
  end
end

This code runs authenticate_user before show action, then runs log_action after it.

Execution Table

Step	Filter/Action	When Run	Effect	Notes
1	authenticate_user	Before action	Checks user login	Prevents action if not logged in
2	show	Action	Displays post	Main controller method
3	log_action	After action	Logs action details	Runs after action, before response sent
4	Response sent	After filters done	Sends page to user	End of request cycle

💡 Request ends after after_action filters run and response is sent

Variable Tracker

Variable	Start	After before_action	After action	After after_action	Final
user_authenticated	nil	true or false	true or false	true or false	true or false
response_ready	false	false	true	true	true
log_written	false	false	false	true	true

Key Moments - 3 Insights

Why does the before_action run before the controller method?

Because Rails runs all before_action filters first to prepare or check things before the main action runs, as shown in execution_table step 1 before step 2.

Can after_action filters change the response sent to the user?

Yes, after_action filters run after the action but before the response is sent, so they can modify the response (e.g., headers), mainly for logging or cleanup, as shown in execution_table step 3 before step 4.

What happens if a before_action filter stops the request?

If a before_action filter halts the request (like redirecting), the controller action and after_action filters do not run, stopping at that filter.

Visual Quiz - 3 Questions

Look at the execution_table, at which step does the main controller action run?

A Step 1

B Step 2

C Step 3

D Step 4

💡 Hint

Check the 'Filter/Action' column for the controller action name.

According to variable_tracker, when does 'log_written' become true?

A After after_action

B After action

C After before_action

D At start

💡 Hint

Look at the 'log_written' row and see when it changes to true.

If the before_action filter fails authentication, what happens next?

A Controller action runs anyway

B After_action runs but action does not

C Request stops before action and after_action

D Response is sent before filters

💡 Hint

Recall key_moments about before_action halting the request.

Concept Snapshot

Rails filters run in order:
- before_action: runs before controller method, used for checks
- controller action: main code for request
- after_action: runs after action, used for logging or cleanup
Filters help organize code around actions cleanly.

Full Transcript

In Rails, before_action filters run first to prepare or check things before the main controller action runs. Then the controller action executes to handle the request. After that, after_action filters run for tasks like logging. Finally, the response is sent to the user. If a before_action filter stops the request, the action and after_action filters do not run. Variables like user authentication status change during before_action, response readiness after the action, and logs are written after after_action. This flow helps keep controller code clean and organized.