Error messages and display in Ruby on Rails - Step-by-Step Execution

Concept Flow - Error messages and display

User submits form

↓

Controller receives data

↓

Model validates data

↓

Save data

↓

Redirect or render success

↓

User sees error messages on form

This flow shows how Rails handles form submission, validates data, and displays error messages back to the user if needed.

Execution Sample

Ruby on Rails

class User < ApplicationRecord
  validates :email, presence: true
end

# In controller
if @user.save
  redirect_to root_path
else
  render :new
end

This code validates that email is present before saving a user; if validation fails, it re-renders the form to show errors.

Execution Table

Step	Action	Validation Result	Controller Decision	View Rendered	Error Messages Displayed
1	User submits form with email=''	Email presence validation fails	Render :new (form) again	Form with errors	Email can't be blank
2	User submits form with email='[e-mail]'	Validation passes	Redirect to root_path	Success page	No errors

💡 Execution stops after redirect or rendering form with errors based on validation result

Variable Tracker

Variable	Start	After Step 1	After Step 2
@user.email	nil	'' (empty string)	'[e-mail]'
@user.errors.full_messages	[]	["Email can't be blank"]	[]

Key Moments - 2 Insights

Why does the form re-render with errors instead of redirecting when validation fails?

Because in the execution_table row 1, validation fails and the controller chooses to render the form again to show errors instead of redirecting.

How are error messages accessed to display on the form?

Error messages are collected in @user.errors.full_messages as shown in variable_tracker after step 1, then displayed in the view.

Visual Quiz - 3 Questions

Look at the execution_table, what does the controller do when validation fails?

A Save the user anyway

B Redirect to root_path

C Render the form again with errors

D Raise an exception

💡 Hint

See execution_table row 1 under Controller Decision and View Rendered columns

According to variable_tracker, what is the value of @user.errors.full_messages after step 2?

A []

B ["Email can't be blank"]

C nil

D ['Invalid email']

💡 Hint

Check variable_tracker row for @user.errors.full_messages after step 2

If the user submits the form with a valid email, what will happen according to execution_table?

A Form re-renders with error messages

B User is redirected to root_path

C Validation fails

D Error messages are shown

💡 Hint

Look at execution_table row 2 under Validation Result and Controller Decision

Concept Snapshot

Rails error messages flow:
- User submits form
- Model validates data
- If errors, controller renders form with @model.errors
- Errors shown in view with full_messages
- If no errors, save and redirect
Use 'validates' in model and check 'save' result in controller.

Full Transcript

In Rails, when a user submits a form, the controller receives the data and tries to save the model. The model runs validations like checking if email is present. If validation fails, the controller renders the form again, passing error messages collected in @model.errors. The view displays these errors so the user knows what to fix. If validation passes, the controller redirects to a success page. This flow helps users correct mistakes easily by showing clear error messages on the form.