DATAMONGO-2047 - Polishing.

Retain previous options when calling withTimezone(…)/onNull…(…). Add tests. Javadoc.

Original pull request: #593.

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/aggregation/DateOperators.java

@@ -1409,7 +1409,7 @@ public class DateOperators {
 		public DateToString withTimezone(Timezone timezone) {
 
 			Assert.notNull(timezone, "Timezone must not be null.");
-			return new DateToString(argumentMap(get("date"), get("format"), timezone));
+			return new DateToString(append("timezone", timezone));
 		}
 
 		/**
@@ -1455,7 +1455,7 @@ public class DateOperators {
 
 		private static java.util.Map<String, Object> argumentMap(Object date, @Nullable String format, Timezone timezone) {
 
-			java.util.Map<String, Object> args = new LinkedHashMap<String, Object>(2);
+			java.util.Map<String, Object> args = new LinkedHashMap<>(2);
 
 			if (StringUtils.hasText(format)) {
 				args.put("format", format);
@@ -1469,6 +1469,25 @@ public class DateOperators {
 			return args;
 		}
 
+		protected java.util.Map<String, Object> append(String key, Object value) {
+
+			java.util.Map<String, Object> clone = new LinkedHashMap<>(argumentMap());
+
+			if (value instanceof Timezone) {
+
+				if (ObjectUtils.nullSafeEquals(value, Timezone.none())) {
+					clone.remove("timezone");
+				} else {
+					clone.put("timezone", ((Timezone) value).value);
+				}
+
+			} else {
+				clone.put(key, value);
+			}
+
+			return clone;
+		}
+
 		public interface FormatBuilder {
 
 			/**

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/aggregation/ProjectionOperation.java

@@ -1207,7 +1207,7 @@ public class ProjectionOperation implements FieldsExposingAggregationOperation {
 		/**
 		 * Generates a {@code $dateToString} expression that takes the date representation of the previously mentioned field
 		 * using the server default format. <br />
-		 * strong>NOTE:</strong> Requires MongoDB 4.0 or later.
+		 * <strong>NOTE:</strong> Requires MongoDB 4.0 or later.
 		 *
 		 * @return
 		 * @since 2.1

spring-data-mongodb/src/test/java/org/springframework/data/mongodb/core/aggregation/ProjectionOperationUnitTests.java

@@ -1335,7 +1335,7 @@ public class ProjectionOperationUnitTests {
 				Document.parse("{ $project: { time: { $dateToString: { format: \"%H:%M:%S:%L\", date: \"$date\" } } } }"));
 	}
 
-	@Test // DATAMONGO-1834
+	@Test // DATAMONGO-1834, DATAMONGO-2047
 	public void shouldRenderDateToStringAggregationExpressionWithTimezone() {
 
 		Document agg = project()
@@ -1344,6 +1344,13 @@ public class ProjectionOperationUnitTests {
 
 		assertThat(agg).isEqualTo(Document.parse(
 				"{ $project: { time: { $dateToString: { format: \"%H:%M:%S:%L\", date: \"$date\", \"timezone\" : \"America/Chicago\" } } } } } }"));
+
+		Document removedTimezone = project().and(DateOperators.dateOf("date")
+				.withTimezone(Timezone.valueOf("America/Chicago")).toString("%H:%M:%S:%L").withTimezone(Timezone.none()))
+				.as("time").toDocument(Aggregation.DEFAULT_CONTEXT);
+
+		assertThat(removedTimezone).isEqualTo(
+				Document.parse("{ $project: { time: { $dateToString: { format: \"%H:%M:%S:%L\", date: \"$date\" } } } } } }"));
 	}
 
 	@Test // DATAMONGO-2047
@@ -1357,6 +1364,29 @@ public class ProjectionOperationUnitTests {
 				.parse("{ $project: { time: { $dateToString: { date: \"$date\", \"onNull\" : \"$fallback-field\" } } } }"));
 	}
 
+	@Test // DATAMONGO-2047
+	public void shouldRenderDateToStringWithOnNullExpression() {
+
+		Document agg = project()
+				.and(DateOperators.dateOf("date").toStringWithDefaultFormat()
+						.onNullReturnValueOf(LiteralOperators.valueOf("my-literal").asLiteral()))
+				.as("time").toDocument(Aggregation.DEFAULT_CONTEXT);
+
+		assertThat(agg).isEqualTo(Document.parse(
+				"{ $project: { time: { $dateToString: { date: \"$date\", \"onNull\" : { \"$literal\": \"my-literal\"} } } } }"));
+	}
+
+	@Test // DATAMONGO-2047
+	public void shouldRenderDateToStringWithOnNullAndTimezone() {
+
+		Document agg = project().and(DateOperators.dateOf("date").toStringWithDefaultFormat()
+				.onNullReturnValueOf("fallback-field").withTimezone(Timezone.ofField("foo"))).as("time")
+				.toDocument(Aggregation.DEFAULT_CONTEXT);
+
+		assertThat(agg).isEqualTo(Document.parse(
+				"{ $project: { time: { $dateToString: { date: \"$date\", \"onNull\" : \"$fallback-field\", \"timezone\": \"$foo\" } } } }"));
+	}
+
 	@Test // DATAMONGO-1536
 	public void shouldRenderSumAggregationExpression() {