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

@@ -1401,7 +1401,7 @@ public class DateOperators {
 		public DateToString withTimezone(Timezone timezone) {
 
 			Assert.notNull(timezone, "Timezone must not be null.");
-			return new DateToString(argumentMap(get("date"), (String) get("format"), timezone));
+			return new DateToString(append("timezone", timezone));
 		}
 
 		/**
@@ -1461,6 +1461,25 @@ public class DateOperators {
 			return args;
 		}
 
+		protected java.util.Map<String, Object> append(String key, Object value) {
+
+			java.util.Map<String, Object> clone = new LinkedHashMap<String, Object>(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

@@ -1213,7 +1213,7 @@ 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 1.10.15

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

@@ -1301,7 +1301,7 @@ public class ProjectionOperationUnitTests {
 				is(JSON.parse("{ $project: { time: { $dateToString: { format: \"%H:%M:%S:%L\", date: \"$date\" } } } }")));
 	}
 
-	@Test // DATAMONGO-1834
+	@Test // DATAMONGO-1834, DATAMONGO-2047
 	public void shouldRenderDateToStringAggregationExpressionWithTimezone() {
 
 		DBObject agg = project()
@@ -1310,6 +1310,13 @@ public class ProjectionOperationUnitTests {
 
 		assertThat(agg, is(JSON.parse(
 				"{ $project: { time: { $dateToString: { format: \"%H:%M:%S:%L\", date: \"$date\", \"timezone\" : \"America/Chicago\" } } } } } }")));
+
+		DBObject removedTimezone = project().and(DateOperators.dateOf("date")
+				.withTimezone(Timezone.valueOf("America/Chicago")).toString("%H:%M:%S:%L").withTimezone(Timezone.none()))
+				.as("time").toDBObject(Aggregation.DEFAULT_CONTEXT);
+
+		assertThat(removedTimezone,
+				is(JSON.parse("{ $project: { time: { $dateToString: { format: \"%H:%M:%S:%L\", date: \"$date\" } } } } } }")));
 	}
 
 	@Test // DATAMONGO-2047
@@ -1323,6 +1330,29 @@ public class ProjectionOperationUnitTests {
 				.parse("{ $project: { time: { $dateToString: { date: \"$date\", \"onNull\" : \"$fallback-field\" } } } }")));
 	}
 
+	@Test // DATAMONGO-2047
+	public void shouldRenderDateToStringWithOnNullExpression() {
+
+		DBObject agg = project()
+				.and(DateOperators.dateOf("date").toStringWithDefaultFormat()
+						.onNullReturnValueOf(LiteralOperators.valueOf("my-literal").asLiteral()))
+				.as("time").toDBObject(Aggregation.DEFAULT_CONTEXT);
+
+		assertThat(agg, is(JSON.parse(
+				"{ $project: { time: { $dateToString: { date: \"$date\", \"onNull\" : { \"$literal\": \"my-literal\"} } } } }")));
+	}
+
+	@Test // DATAMONGO-2047
+	public void shouldRenderDateToStringWithOnNullAndTimezone() {
+
+		DBObject agg = project().and(DateOperators.dateOf("date").toStringWithDefaultFormat()
+				.onNullReturnValueOf("fallback-field").withTimezone(Timezone.ofField("foo"))).as("time")
+				.toDBObject(Aggregation.DEFAULT_CONTEXT);
+
+		assertThat(agg, is(JSON.parse(
+				"{ $project: { time: { $dateToString: { date: \"$date\", \"onNull\" : \"$fallback-field\", \"timezone\": \"$foo\" } } } }")));
+	}
+
 	@Test // DATAMONGO-1536
 	public void shouldRenderSumAggregationExpression() {