MONGOID-5890 Handle BSON::Decimal128 specially when checking for changes

lib/mongoid/attributes.rb

@@ -175,7 +175,7 @@ def write_attribute(name, value)
           localized = fields[field_name].try(:localized?)
           attributes_before_type_cast[name.to_s] = value
           typed_value = typed_value_for(field_name, value)
-          unless attributes[field_name] == typed_value || attribute_changed?(field_name)
+          unless attribute_will_not_change?(field_name, typed_value) || attribute_changed?(field_name)
             attribute_will_change!(field_name)
           end
           if localized
@@ -366,5 +366,23 @@ def lookup_attribute_presence(name, value)
       end
       value.present?
     end
+
+    # If `value` is a `BSON::Decimal128`, convert it to a `BigDecimal` for
+    # comparison purposes. This is necessary because `BSON::Decimal128` does
+    # not implement `#==` in a way that is compatible with `BigDecimal`.
+    def normalize_value(value)
+      value.is_a?(BSON::Decimal128) ? BigDecimal(value.to_s) : value
+    end
+
+    # Determine if the attribute will not change, by comparing the current
+    # value with the new value. The values are normalized to account for
+    # types that do not implement `#==` in a way that is compatible with
+    # each other, such as `BSON::Decimal128` and `BigDecimal`.
+    def attribute_will_not_change?(field_name, typed_value)
+      normalized_attribute = normalize_value(attributes[field_name])
+      normalized_typed_value = normalize_value(typed_value)
+
+      normalized_attribute == normalized_typed_value
+    end
   end
 end

spec/mongoid/attributes_spec.rb

@@ -1729,6 +1729,19 @@
         end
       end
     end
+
+    context 'when map_big_decimal_to_decimal128 is enabled' do
+      config_override :map_big_decimal_to_decimal128, true
+
+      context 'when writing an identical number' do
+        let(:band) { Band.create!(name: 'Nirvana', sales: 123456.78).reload }
+
+        it 'does not mark the document as changed' do
+          band.sales = 123456.78
+          expect(band.changed?).to be false
+        end
+      end
+    end
   end
 
   describe "#typed_value_for" do