DimensionsValidator to use for images uploaded using Paperclip.

dimensions_validator.rb

class DimensionsValidator < ActiveModel::EachValidator
  def validate_each(record, attribute, value)
    temp_file = value.queued_for_write[:original]
    return unless temp_file.present?
    return unless geo = Paperclip::Geometry.from_file(temp_file)

    options.slice(:width, :height).each_pair do |key, expectation|
      next unless dimension = geo.send(key)

      case expectation
      when Integer
        record.errors[attribute] << "#{key} must be exactly #{expectation}px" unless dimension == expectation
      when Range
        unless dimension.in? expectation
          record.errors[attribute] << "#{key} must be between #{expectation.min} and #{expectation.max} px"
        end
      when String
        if expectation =~ /^([<>]=?)(\d+)$/
          # we directly pass the comparator string as a method call
          unless dimension.send($1, $2.to_i)
            human_error = case $1
            when '<'  then 'smaller than'
            when '<=' then 'smaller or equal than'
            when '>'  then 'greater than'
            when '>=' then 'at least'
            end
            record.errors[attribute] << "#{key} must be #{human_error} #{$2}px"
          end
        else
          raise ArgumentError, "Incorrect syntax for given #{key} argument"
        end
      end
    end
  end
end