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How to Build Custom ActiveRecord Validations

September 20, 2021

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Your Teacher

Hi, I'm Chris. I'm the creator of GoRails, Hatchbox.io and Jumpstart. I spend my time creating tutorials and tools to help Ruby on Rails developers build apps better and faster.

About This Episode

Custom ActiveRecord validations are easy to make and they're implemented exactly the same way as the built-in Rails validations. It's super handy to be able to add nice, clean validations that you can repeat throughout your apps.



Here's our example Resizable Image Validator that validates image content types that are uploaded. We can make sure we only accept image types that are resizable using this validator.

# app/validators/resizable_image_validator.rb
class ResizableImageValidator < ActiveModel::EachValidator
  # Only allow resizable image formats
  # SVGs aren't allowed because they can contain XSS
  def validate_each(record, attribute, value)
    return unless value.attached?

    if ActiveStorage.variable_content_types.exclude?(value.content_type)
      record.errors.add(attribute, :image_format_not_supported)
# app/models/user.rb
class User < ApplicationRecord
  has_one_attached :avatar
  validates :avatar, resizable_image: true

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