Presence validation in Ruby on Rails - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is presence validation in Rails?

Presence validation ensures that a specific attribute is not empty or nil before saving a record in the database.

beginner

How do you add presence validation to a model attribute in Rails?

Use validates :attribute_name, presence: true inside the model class.

beginner

What happens if a presence validation fails when saving a record?

The record is not saved, and error messages are added to the model's errors collection.

intermediate

Can presence validation check multiple attributes at once?

Yes, you can validate multiple attributes by listing them: validates :name, :email, presence: true.

beginner

Why is presence validation important in web applications?

It prevents saving incomplete or invalid data, ensuring the app works correctly and data stays reliable.

Which Rails method is used to enforce presence validation on a model attribute?

A validates :attribute, presence: true

B validate_presence_of :attribute

C check_presence :attribute

D require :attribute

The correct syntax is validates :attribute, presence: true to enforce presence validation.

What will happen if a model fails presence validation when calling save?

A The record will save anyway

B The attribute will be auto-filled

C The app will crash

D The record will not save and errors will be added

If presence validation fails, the record is not saved and error messages are added to the model.

Can you validate presence for multiple attributes in one line?

A Yes, but only for string attributes

B No, you must validate each attribute separately

C Yes, by listing attributes like <code>validates :name, :email, presence: true</code>

D No, Rails does not support multiple attributes in presence validation

Rails allows validating presence for multiple attributes by listing them in one validates call.

Which of these values will fail presence validation?

A "Hello"

B " " (a space)

C nil

D 0

Presence validation fails if the attribute is nil or blank (including whitespace-only strings like " "). "Hello" and 0 are present.

Where do you place presence validation code in a Rails app?

A In the model

B In the database schema

C In the view

D In the controller

Presence validation is declared inside the model class to check data before saving.

Explain how to add presence validation to a Rails model and what it does.

Think about where validations go and what presence means.

Why is presence validation useful in a web application? Give an example.

Consider what happens if important info is missing.

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