Custom validation methods in Ruby on Rails - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a custom validation method in Rails?

A custom validation method is a user-defined method inside a Rails model that checks if an object meets specific rules beyond built-in validations. It helps ensure data is correct before saving.

beginner

How do you tell Rails to use a custom validation method?

You use the validate :method_name syntax inside the model. Rails then calls that method during validation.

beginner

What should a custom validation method do if the data is invalid?

It should add an error message to the model's errors collection using errors.add(:attribute, "message"). This prevents saving and shows the problem.

intermediate

Why use custom validation methods instead of built-in validators?

Custom validations let you check complex or unique rules that built-in validators can't handle, like comparing two fields or checking external conditions.

beginner

Example: How to write a custom validation method that ensures a user's age is at least 18?

Inside the User model, define a method like:<br>

def adult_age
  errors.add(:age, "must be at least 18") if age && age < 18
end
validate :adult_age

Which keyword tells Rails to run a custom validation method?

A validates_with

B validates

C validate

D validate_with

Use validate :method_name to run a custom validation method.

Where do you add error messages inside a custom validation method?

A add_error(:attribute, "message")

B errors.add(:attribute, "message")

C error.add(:attribute, "message")

D add.errors(:attribute, "message")

The correct syntax is errors.add(:attribute, "message").

What happens if a custom validation adds an error to the model?

A The model deletes itself

B The model saves anyway

C The error is ignored

D The model will not save

Adding errors prevents the model from saving until fixed.

Which of these is a reason to use a custom validation?

A To check a rule built-in validators can't handle

B To speed up saving records

C To automatically fix errors

D To skip validations

Custom validations handle special rules not covered by built-in ones.

How do you call a custom validation method named check_name?

A validate :check_name

B validates :check_name

C validate_with :check_name

D validates_with :check_name

Use validate :check_name to run the method during validation.

Explain how to create and use a custom validation method in a Rails model.

Think about where to put the method and how Rails knows to run it.

Describe why and when you would choose a custom validation method over built-in validators.

Consider what built-in validators can't do.