Implement CreateAutoTimestampConverter (#282)

* Implement CreateAutoTimestampConverter

Implement a JPA-based converter for CreateAutoTimestamp, allowing us to
persist instances of this class.

Note that converters appear to be required to convert to and from database
types that are generally known to JDBC.  For example, conversion to Timestamp
works, conversion to OffsetDateTime does not (even though this works through
the JDBC interface directly).

core/src/main/java/google/registry/persistence/CreateAutoTimestampConverter.java

@@ -0,0 +1,50 @@
+// Copyright 2019 The Nomulus Authors. All Rights Reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+package google.registry.persistence;
+
+import static com.google.common.base.MoreObjects.firstNonNull;
+import static google.registry.model.transaction.TransactionManagerFactory.jpaTm;
+
+import google.registry.model.CreateAutoTimestamp;
+import google.registry.util.DateTimeUtils;
+import java.sql.Timestamp;
+import java.time.ZoneOffset;
+import java.time.ZonedDateTime;
+import javax.annotation.Nullable;
+import javax.persistence.AttributeConverter;
+import javax.persistence.Converter;
+import org.joda.time.DateTime;
+
+/** JPA converter to for storing/retrieving CreateAutoTimestamp objects. */
+@Converter
+public class CreateAutoTimestampConverter
+    implements AttributeConverter<CreateAutoTimestamp, Timestamp> {
+
+  @Override
+  public Timestamp convertToDatabaseColumn(CreateAutoTimestamp entity) {
+    DateTime dateTime =
+        firstNonNull(((CreateAutoTimestamp) entity).getTimestamp(), jpaTm().getTransactionTime());
+    return Timestamp.from(DateTimeUtils.toZonedDateTime(dateTime).toInstant());
+  }
+
+  @Override
+  @Nullable
+  public CreateAutoTimestamp convertToEntityAttribute(@Nullable Timestamp columnValue) {
+    if (columnValue == null) {
+      return null;
+    }
+    ZonedDateTime zdt = ZonedDateTime.ofInstant(columnValue.toInstant(), ZoneOffset.UTC);
+    return CreateAutoTimestamp.create(DateTimeUtils.toJodaDateTime(zdt));
+  }
+}