Uniqueness validation in Ruby on Rails - Step-by-Step Execution

Concept Flow - Uniqueness validation

Start: New record input

↓

Check if value exists in DB?

↓

Add error

↓

Reject save

↓

End

Rails checks if the value already exists in the database before saving. If it does, it adds an error and rejects saving. Otherwise, it saves the record.

Execution Sample

Ruby on Rails

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

This code ensures that each User's email is unique before saving to the database.

Execution Table

Step	Input Email	DB Check Result	Validation Result	Action
1	"[e-mail]"	Not found	Valid	Record saved
2	"[e-mail]"	Found	Invalid	Save rejected with error
3	"[e-mail]"	Not found	Valid	Record saved

💡 Execution stops when record is saved or rejected based on uniqueness check.

Variable Tracker

Variable	Start	After 1	After 2	After 3
email	nil	"[e-mail]"	"[e-mail]"	"[e-mail]"
validation_result	nil	Valid	Invalid	Valid
record_saved	false	true	false	true

Key Moments - 2 Insights

Why does the second record with the same email fail to save?

Because the uniqueness validation checks the database and finds the email already exists (see execution_table step 2), so it marks the record invalid and rejects saving.

Does uniqueness validation check only the current input or the whole database?

It checks the whole database for existing records with the same value before saving, as shown in the DB Check Result column in the execution_table.

Visual Quiz - 3 Questions

Look at the execution table, what is the validation result at step 2?

A Invalid

B Not checked

C Valid

D Error

💡 Hint

Check the 'Validation Result' column at step 2 in the execution_table.

At which step does the record get saved with a new email?

A Step 1

B Step 2

C Step 3

D No record is saved

💡 Hint

Look at the 'Action' column in the execution_table for when 'Record saved' happens with a new email.

If the email at step 3 was changed to "[e-mail]", what would happen?

A Record saved

B Save rejected with error

C Validation skipped

D Database error

💡 Hint

Refer to step 2 where the same email caused save rejection due to uniqueness validation.

Concept Snapshot

Uniqueness validation in Rails:
- Use validates :field, uniqueness: true
- Checks DB for existing value before save
- If found, adds error and rejects save
- Ensures no duplicate values in that field
- Works automatically on save/update

Full Transcript

Uniqueness validation in Rails ensures that a field's value is unique in the database before saving a record. When you try to save a record, Rails checks if the value already exists. If it does, it adds an error and prevents saving. If not, it saves the record. For example, validating uniqueness of email means no two users can have the same email. The execution table shows three steps: first saving a new email succeeds, second saving the same email fails, and third saving a different email succeeds. Variables like email, validation result, and record saved change accordingly. This helps prevent duplicate data in your app.