Fix AddFieldsOperationBuilder to treat String value as Field reference

This commit modifies the AddFieldsOperationBuilder to correctly treat String values as field references.
When a String value is passed, it is now interpreted as a reference to another field, following MongoDB's field reference syntax.

Closes spring-projects#4933

Signed-off-by: kssumin <201566@jnu.ac.kr>

Author: kssumin

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

@@ -31,6 +31,7 @@
  * </pre>
  *
  * @author Christoph Strobl
+ * @author Kim Sumin
  * @since 3.0
  * @see <a href="https://docs.mongodb.com/manual/reference/operator/aggregation/addFields/">MongoDB Aggregation
  *      Framework: $addFields</a>
@@ -148,7 +149,7 @@ public AddFieldsOperationBuilder withValue(Object value) {
 				@Override
 				public AddFieldsOperationBuilder withValueOf(Object value) {
 
-					valueMap.put(field, value instanceof String stringValue ? Fields.fields(stringValue) : value);
+					valueMap.put(field, value instanceof String stringValue ? Fields.field(stringValue) : value);
 					return AddFieldsOperationBuilder.this;
 				}
 

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

@@ -33,6 +33,7 @@
  *
  * @author Christoph Strobl
  * @author Mark Paluch
+ * @author Kim Sumin
  */
 class AddFieldsOperationUnitTests {
 
@@ -127,6 +128,22 @@ void exposesFieldsCorrectly() {
 		assertThat(fields.getField("does-not-exist")).isNull();
 	}
 
+	@Test // DATAMONGO-4933
+	void rendersStringValueAsFieldReferenceCorrectly() {
+
+		AddFieldsOperation operation = AddFieldsOperation.builder().addField("name").withValueOf("value").build();
+
+		assertThat(operation.toPipelineStages(contextFor(Scores.class)))
+				.containsExactly(Document.parse("{\"$addFields\" : {\"name\":\"$value\"}}"));
+
+		AddFieldsOperation mappedOperation = AddFieldsOperation.builder().addField("totalHomework").withValueOf("homework")
+				.build();
+
+		assertThat(mappedOperation.toPipelineStages(contextFor(ScoresWithMappedField.class)))
+				.containsExactly(Document.parse("{\"$addFields\" : {\"totalHomework\":\"$home_work\"}}"));
+	}
+
+
 	private static AggregationOperationContext contextFor(@Nullable Class<?> type) {
 
 		if (type == null) {