Validations

Validation checks data validity before it gets saved to the DB.

class Post < ApplicationRecord
  validates :title, presence: true, length: { maximum: 200 }
  validates :content, presence: true
  validates :slug, uniqueness: true
  validates :email, format: { with: URI::MailTo::EMAIL_REGEXP }
  validates :status, inclusion: { in: %w[draft published archived] }
  validate :custom_validation_method

  private

  def custom_validation_method
    if publish_date.present? && publish_date < Date.today
      errors.add(:publish_date, 'cannot be in the past')
    end
  end
end

On validation failure:

• save → returns false

• save! → raises ActiveRecord::RecordInvalid

• model.errors.full_messages to access error messages

• In views, @post.errors[:title] for per-field errors