Handle upload file size limit in UI

lib/file_size_validator.rb

@@ -0,0 +1,65 @@
+class FileSizeValidator < ActiveModel::EachValidator
+  MESSAGES  = { :is => :wrong_size, :minimum => :size_too_small, :maximum => :size_too_big }.freeze
+  CHECKS    = { :is => :==, :minimum => :>=, :maximum => :<= }.freeze
+
+  DEFAULT_TOKENIZER = lambda { |value| value.split(//) }
+  RESERVED_OPTIONS  = [:minimum, :maximum, :within, :is, :tokenizer, :too_short, :too_long]
+
+  def initialize(options)
+    if range = (options.delete(:in) || options.delete(:within))
+      raise ArgumentError, ":in and :within must be a Range" unless range.is_a?(Range)
+      options[:minimum], options[:maximum] = range.begin, range.end
+      options[:maximum] -= 1 if range.exclude_end?
+    end
+
+    super
+  end
+
+  def check_validity!
+    keys = CHECKS.keys & options.keys
+
+    if keys.empty?
+      raise ArgumentError, 'Range unspecified. Specify the :within, :maximum, :minimum, or :is option.'
+    end
+
+    keys.each do |key|
+      value = options[key]
+
+      unless value.is_a?(Integer) && value >= 0
+        raise ArgumentError, ":#{key} must be a nonnegative Integer"
+      end
+    end
+  end
+
+  def validate_each(record, attribute, value)
+    raise(ArgumentError, "A CarrierWave::Uploader::Base object was expected") unless value.kind_of? CarrierWave::Uploader::Base
+    
+    value = (options[:tokenizer] || DEFAULT_TOKENIZER).call(value) if value.kind_of?(String)
+
+    CHECKS.each do |key, validity_check|
+      next unless check_value = options[key]
+
+      value ||= [] if key == :maximum
+
+      value_size = value.size
+      next if value_size.send(validity_check, check_value)
+
+      errors_options = options.except(*RESERVED_OPTIONS)
+      errors_options[:file_size] = help.number_to_human_size check_value
+
+      default_message = options[MESSAGES[key]]
+      errors_options[:message] ||= default_message if default_message
+
+      record.errors.add(attribute, MESSAGES[key], errors_options)
+    end
+  end
+  
+  def help
+    Helper.instance
+  end
+
+  class Helper
+    include Singleton
+    include ActionView::Helpers::NumberHelper
+  end
+end