image size validator

image_size_validator.rb

# Refs: http://stackoverflow.com/questions/7527887/validate-image-size-in-carrierwave-uploader

class ImageSizeValidator < ActiveModel::EachValidator
  def validate_each(record, attribute, value)
    unless value.blank?

      image = MiniMagick::Image.open(value.path)
      checks = [
        { :option => :width,
          :field => :width,
          :function => :'==',
          :message =>"width must be %d px."},
        { :option => :height,
          :field => :height,
          :function => :'==',
          :message =>"height must be %d px."},
        { :option => :max_width,
          :field => :width,
          :function => :'<=',
          :message =>"width must be at most %d px."},
        { :option => :max_height,
          :field => :height,
          :function => :'<=',
          :message =>"height must be at most %d px."},
        { :option => :min_width,
          :field => :width,
          :function => :'>=',
          :message =>"width must be at least %d px."},
        { :option => :min_height,
          :field => :height,
          :function => :'>=',
          :message =>"height must be at least %d px."},
      ]

      checks.each do |p|
        if options.has_key?(p[:option]) and
          !image[p[:field]].send(p[:function], options[p[:option]])
          record.errors[attribute] << p[:message] % options[p[:option]]
        end
      end

    end
  end
end

# between 300px and 800px in width, and 100px and 500px in height.
validates :image, image_size: {min_width: 300, max_width: 800, min_height: 100, max_height: 500}