How can I implement custom validations in Ruby on Rails?

How can I implement custom validations in Ruby on Rails?

You can create custom validations in Rails by defining a method in your model and using the validate method.

Can you give me an example of a custom validation?

Sure! For instance, if you want to validate that a username is unique and starts with a letter, you can do this:

class User < ApplicationRecord
  validate :username_must_start_with_letter

  private

  def username_must_start_with_letter
    unless username =~ /\\A[a-zA-Z]/
      errors.add(:username, 'must start with a letter')
    end
  end
end

What if I want to add multiple validations?

You can chain multiple validations by calling validate multiple times or by combining them in a single method. For example:

class User < ApplicationRecord
  validate :username_must_start_with_letter, :username_must_be_unique

  private

  def username_must_start_with_letter
    # validation logic
  end

  def username_must_be_unique
    # validation logic
  end
end

Is there a way to create reusable custom validations?

Yes! You can create a module with your custom validation methods and include it in any model where you need those validations.