Rails 4: Validate that an image to upload is unique

If you want to verify you already uploaded the same image before, this is an easy way to do it:

1. Create a migration to store the SHA-1 of the image:

rails g migration add_image_sha1_to_my_model image_sha1:string

1. Calculate SHA-1 on the model:

before_validation(on: :create) do
  self.sha1 = Digest::SHA1.file(image.path).hexdigest
end

1. Create an custom validator for the image (it can be used for other images):

class ImageValidator < ActiveModel::Validator
  def validate(image)
    image.errors.add(:image, 'already exists') if image.class.exists?(sha1: image.sha1)
  end
end

1. Add the validation to the model:

class MyModel < ActiveRecord::Base
  validates_with ImageValidator
end