add precision option for datetime string dumping

adding spec

Author: f0ster

lib/dynamoid/dumping.rb

@@ -222,7 +222,11 @@ def format_datetime(value, options)
 
         if use_string_format
           value_in_time_zone = Dynamoid::DynamodbTimeZone.in_time_zone(value)
-          value_in_time_zone.iso8601
+          if options[:iso_precision].nil?
+            value_in_time_zone.iso8601
+          else
+            value_in_time_zone.iso8601(options[:iso_precision].to_i)
+          end
         else
           unless value.respond_to?(:to_i) && value.respond_to?(:nsec)
             value = value.to_time

lib/dynamoid/type_casting.rb

@@ -227,14 +227,22 @@ def process(value)
           nil
         elsif value.is_a?(String)
           dt = begin
+                 # parse first for stricter constraint failure before attempting to parse iso for precision support
+                 # e.g. DateTime.iso8601("2018-12") is valid
                  DateTime.parse(value)
+                 DateTime.iso8601(value)
                rescue StandardError
                  nil
                end
           if dt
-            seconds = string_utc_offset(value) || ApplicationTimeZone.utc_offset
-            offset = seconds_to_offset(seconds)
-            DateTime.new(dt.year, dt.mon, dt.mday, dt.hour, dt.min, dt.sec, offset)
+            dt_has_precision = dt.to_f % 1 != 0
+            if dt_has_precision
+              dt.utc
+            else
+              seconds = string_utc_offset(value) || ApplicationTimeZone.utc_offset
+              offset = seconds_to_offset(seconds)
+              DateTime.new(dt.year, dt.mon, dt.mday, dt.hour, dt.min, dt.sec, offset)
+            end
           end
         else
           value.to_datetime

lib/dynamoid/undumping.rb

@@ -202,7 +202,7 @@ def process(value)
                             else
                               @options[:store_as_string]
                             end
-        value = DateTime.iso8601(value).to_time.to_i if use_string_format
+        value = DateTime.iso8601(value).to_time if use_string_format
         ApplicationTimeZone.at(value)
       end
     end

spec/dynamoid/dumping_spec.rb

@@ -239,6 +239,8 @@
         Dynamoid.config.store_datetime_as_string = store_datetime_as_string
       end
 
+
+
       it 'prioritize field option over global one' do
         store_datetime_as_string = Dynamoid.config.store_datetime_as_string
         Dynamoid.config.store_datetime_as_string = false
@@ -366,6 +368,18 @@
         expect(raw_attributes(obj)[:signed_up_on]).to eql('2017-09-25')
       end
 
+      it 'stores in string format with precision when :iso_precision is present' do
+        klass = new_class do
+          field :signed_up_on, :datetime, store_as_string: true, iso_precision: 6
+        end
+
+        signed_up_on = DateTime.now.utc.iso8601(6)
+
+        obj = klass.create(signed_up_on: signed_up_on)
+
+        expect(reload(obj).signed_up_on.to_f).to eql(DateTime.iso8601(signed_up_on).to_f)
+      end
+
       it 'stores in string format when global option :store_date_as_string is true' do
         klass = new_class do
           field :signed_up_on, :date