application record: add array_attribute method.

Add `array_attribute` method that defines helper methods for converting
array attributes to or from strings.

app/models/application_record.rb

@@ -269,6 +269,37 @@ class ApplicationRecord < ActiveRecord::Base
     end
   end
 
+  concerning :AttributeMethods do
+    class_methods do
+      # Defines `<attribute>_string`, `<attribute>_string=`, and `<attribute>=`
+      # methods for converting an array attribute to or from a string.
+      #
+      # The `<attribute>=` setter parses strings into an array using the
+      # `parse` regex. The resulting strings can be converted to another type
+      # with the `cast` option.
+      def array_attribute(name, parse: /[^[:space:]]+/, cast: :itself)
+        define_method "#{name}_string" do
+          send(name).join(" ")
+        end
+
+        define_method "#{name}_string=" do |value|
+          raise ArgumentError, "#{name} must be a String" unless value.respond_to?(:to_str)
+          send("#{name}=", value)
+        end
+
+        define_method "#{name}=" do |value|
+          if value.respond_to?(:to_str)
+            super value.to_str.scan(parse).map(&cast)
+          elsif value.respond_to?(:to_a)
+            super value.to_a
+          else
+            raise ArgumentError, "#{name} must be a String or an Array"
+          end
+        end
+      end
+    end
+  end
+
   def warnings
     @warnings ||= ActiveModel::Errors.new(self)
   end