require 'active_support/core_ext/module/method_transplanting'

module ActiveRecord
  module AttributeMethods
    module Read
      ReaderMethodCache = Class.new(AttributeMethodCache) {
        private
        # We want to generate the methods via module_eval rather than
        # define_method, because define_method is slower on dispatch.
        # Evaluating many similar methods may use more memory as the instruction
        # sequences are duplicated and cached (in MRI).  define_method may
        # be slower on dispatch, but if you're careful about the closure
        # created, then define_method will consume much less memory.
        #
        # But sometimes the database might return columns with
        # characters that are not allowed in normal method names (like
        # 'my_column(omg)'. So to work around this we first define with
        # the __temp__ identifier, and then use alias method to rename
        # it to what we want.
        #
        # We are also defining a constant to hold the frozen string of
        # the attribute name. Using a constant means that we do not have
        # to allocate an object on each call to the attribute method.
        # Making it frozen means that it doesn't get duped when used to
        # key the @attributes in read_attribute.
        def method_body(method_name, const_name)
          <<-EOMETHOD
          def #{method_name}
            name = ::ActiveRecord::AttributeMethods::AttrNames::ATTR_#{const_name}
            read_attribute(name) { |n| missing_attribute(n, caller) }
          end
          EOMETHOD
        end
      }.new

      extend ActiveSupport::Concern

      module ClassMethods
        [:cache_attributes, :cached_attributes, :cache_attribute?].each do |method_name|
          define_method method_name do |*|
            cached_attributes_deprecation_warning(method_name)
            true
          end
        end

        protected

        def cached_attributes_deprecation_warning(method_name)
          ActiveSupport::Deprecation.warn(<<-MESSAGE.strip_heredoc)
            Calling `#{method_name}` is no longer necessary. All attributes are cached.
          MESSAGE
        end

        if Module.methods_transplantable?
          def define_method_attribute(name)
            method = ReaderMethodCache[name]
            generated_attribute_methods.module_eval { define_method name, method }
          end
        else
          def define_method_attribute(name)
            safe_name = name.unpack('h*').first
            temp_method = "__temp__#{safe_name}"

            ActiveRecord::AttributeMethods::AttrNames.set_name_cache safe_name, name

            generated_attribute_methods.module_eval <<-STR, __FILE__, __LINE__ + 1
              def #{temp_method}
                name = ::ActiveRecord::AttributeMethods::AttrNames::ATTR_#{safe_name}
                read_attribute(name) { |n| missing_attribute(n, caller) }
              end
            STR

            generated_attribute_methods.module_eval do
              alias_method name, temp_method
              undef_method temp_method
            end
          end
        end
      end

      # Returns the value of the attribute identified by <tt>attr_name</tt> after
      # it has been typecast (for example, "2004-12-12" in a date column is cast
      # to a date object, like Date.new(2004, 12, 12)).
      def read_attribute(attr_name, &block)
        name = attr_name.to_s
        name = self.class.primary_key if name == 'id'
        @attributes.fetch_value(name, &block)
      end

      private

      def attribute(attribute_name)
        read_attribute(attribute_name)
      end
    end
  end
end