Presence validation in Ruby on Rails - Mini Project: Build & Apply

Presence Validation in Rails Model

📖 Scenario: You are building a simple Rails application to manage a list of books. Each book must have a title and an author. To ensure data quality, you want to make sure that no book can be saved without these details.

🎯 Goal: Create a Rails model Book with presence validation for the title and author attributes. This will prevent saving books without these required fields.

📋 What You'll Learn

Create a Book model with title and author attributes

Add presence validation for title and author

Use Rails validation syntax correctly

Ensure the model is ready to reject invalid records missing these fields

💡 Why This Matters

🌍 Real World

Presence validation is essential in web apps to ensure users provide required information before saving data, preventing incomplete or invalid records.

💼 Career

Understanding and implementing validations is a fundamental skill for Rails developers to maintain data integrity and build reliable applications.

1

Create the Book model with attributes

Create a Rails model called Book with string attributes title and author using attr_accessor.

Ruby on Rails

# Define the Book class with attr_accessor for title and author
# Your code here

Hint

Use attr_accessor to create getter and setter methods for title and author.

2

Add ActiveModel inclusion and validation setup

Include ActiveModel::Model in the Book class to enable validations. Then, add a validation for presence of title and author using validates.

Ruby on Rails

class Book
  attr_accessor :title, :author
  # Include ActiveModel::Model and add presence validation
  # Your code here
end

Hint

Use include ActiveModel::Model to add validation features. Then use validates :attribute, presence: true for each attribute.

3

Create a new Book instance with missing title

Create a new instance of Book called book with author set to 'Jane Austen' and title set to nil.

Ruby on Rails

class Book
  include ActiveModel::Model
  attr_accessor :title, :author
  validates :title, presence: true
  validates :author, presence: true
end

# Create a Book instance with author 'Jane Austen' and no title
# Your code here

Hint

Use Book.new with keyword arguments to set title to nil and author to 'Jane Austen'.

4

Check if the book instance is valid

Call book.valid? to check if the book instance passes the presence validations.

Ruby on Rails

class Book
  include ActiveModel::Model
  attr_accessor :title, :author
  validates :title, presence: true
  validates :author, presence: true
end

book = Book.new(title: nil, author: 'Jane Austen')

# Check if book is valid
# Your code here

Hint

Use the valid? method on the book instance to trigger validations.

Practice

1. What does validates :name, presence: true do in a Rails model?

easy

A. It ensures the name field is not empty before saving.

B. It automatically fills the name field with a default value.

C. It deletes the record if name is empty.

D. It allows name to be empty without errors.

Solution

1. Step 1: Understand presence validation purpose

   Presence validation checks that a field is not blank before saving a record.

2. Step 2: Apply to name field

   Using validates :name, presence: true means the name must have a value.

3. Final Answer:

   It ensures the name field is not empty before saving. -> Option A

4. Quick Check:

   Presence validation = field not empty [OK]

Hint: Presence validation means field cannot be blank [OK]

Common Mistakes:

• Thinking it fills default values automatically
• Confusing presence with uniqueness validation
• Assuming it deletes records if empty

2. Which of the following is the correct syntax to add presence validation for the email attribute in a Rails model?

easy

A. validates_presence_of email

B. validates :email, presence: true

C. validate :email, presence: true

D. validates :email, required: true

Solution

1. Step 1: Recall modern Rails validation syntax

   Rails uses validates :attribute, presence: true for presence validation.

2. Step 2: Check each option

   validates_presence_of email is invalid syntax (missing symbol ":email"); validates :email, presence: true is the correct modern syntax; validate :email, presence: true is incorrect method name; validates :email, required: true uses wrong option key.

3. Final Answer:

   validates :email, presence: true -> Option B

4. Quick Check:

   Use validates :field, presence: true [OK]

Hint: Use validates :field, presence: true for presence [OK]

Common Mistakes:

• Using validate instead of validates
• Using required instead of presence
• Confusing old and new syntax

3. Given this model code:

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

What happens if you try to save User.new(username: '')?

medium

A. The save fails because username is blank.

B. The user is saved with an empty username.

C. An error is raised due to syntax error.

D. The username is automatically set to 'default'.

Solution

1. Step 1: Understand presence validation effect on blank string

   Presence validation treats empty strings as invalid, so saving fails.

2. Step 2: Apply to username: ''

   Since username is empty string, validation fails and save returns false.

3. Final Answer:

   The save fails because username is blank. -> Option A

4. Quick Check:

   Empty string fails presence validation [OK]

Hint: Empty string fails presence validation, save returns false [OK]

Common Mistakes:

• Thinking empty string passes presence validation
• Expecting automatic default value assignment
• Confusing validation failure with exceptions

4. Identify the error in this model validation code:

class Product < ApplicationRecord
  validates :title, presence
end

medium

A. No error, code is correct.

B. Should use validate instead of validates.

C. The attribute title must be a string.

D. Missing true after presence.

Solution

1. Step 1: Check presence validation syntax

   Presence validation requires presence: true, not just presence.

2. Step 2: Identify missing true

   The code misses : true after presence, causing syntax error.

3. Final Answer:

   Missing true after presence. -> Option D

4. Quick Check:

   Presence validation needs presence: true [OK]

Hint: Always write presence: true for presence validation [OK]

Common Mistakes:

• Omitting : true after presence
• Using validate instead of validates
• Assuming attribute type affects validation syntax

5. You want to ensure a Post model's title and content are present, but content can be blank if title is 'Draft'. Which validation setup achieves this?

hard

A. validates :title, presence: true
validates :content, presence: false, unless: -> { title == 'Draft' }

B. validates :title, presence: true
validates :content, presence: true, if: -> { title == 'Draft' }

C. validates :title, presence: true
validates :content, presence: true, unless: -> { title == 'Draft' }

D. validates :title, presence: true
validates :content, presence: false, if: -> { title == 'Draft' }

Solution

1. Step 1: Understand requirement for content

   Content must be present except when title is 'Draft'.

2. Step 2: Use conditional presence validation

   Use presence: true with unless condition to skip validation if title is 'Draft'.

3. Step 3: Check options

   validates :title, presence: true validates :content, presence: true, unless: -> { title == 'Draft' } correctly uses unless: -> { title == 'Draft' } to skip content presence validation when title is 'Draft'. Others misuse if or set presence false incorrectly.

4. Final Answer:

   validates :title, presence: true validates :content, presence: true, unless: -> { title == 'Draft' } -> Option C

5. Quick Check:

   Use unless to skip presence validation conditionally [OK]

Hint: Use unless to skip presence validation conditionally [OK]

Common Mistakes:

• Using if instead of unless for skipping validation
• Setting presence: false which disables validation
• Not combining validations properly