Fix SetOperation.toValueOf rendering for field references.

Use single field reference instead of invalid multi field.

See: #4933

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

@@ -140,7 +140,7 @@ public class SetOperation extends DocumentEnhancingOperation {
 				@Override
 				public SetOperation toValueOf(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 FieldAppender.this.build();
 				}
 

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

@@ -128,7 +128,7 @@ class AddFieldsOperationUnitTests {
 		assertThat(fields.getField("does-not-exist")).isNull();
 	}
 
-	@Test // DATAMONGO-4933
+	@Test // GH-4933
 	void rendersStringValueAsFieldReferenceCorrectly() {
 
 		AddFieldsOperation operation = AddFieldsOperation.builder().addField("name").withValueOf("value").build();
@@ -143,7 +143,6 @@ class AddFieldsOperationUnitTests {
 				.containsExactly(Document.parse("{\"$addFields\" : {\"totalHomework\":\"$home_work\"}}"));
 	}
 
-
 	private static AggregationOperationContext contextFor(@Nullable Class<?> type) {
 
 		if (type == null) {

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

@@ -21,7 +21,6 @@ import java.util.List;
 
 import org.bson.Document;
 import org.junit.jupiter.api.Test;
-
 import org.springframework.data.mongodb.core.convert.MappingMongoConverter;
 import org.springframework.data.mongodb.core.convert.NoOpDbRefResolver;
 import org.springframework.data.mongodb.core.convert.QueryMapper;
@@ -106,6 +105,22 @@ class SetOperationUnitTests {
 						.containsExactly(Document.parse("{\"$set\" : {\"totalHomework\": { \"$sum\" : \"$homework\" }}}"));
 	}
 
+	@Test // GH-4933
+	void rendersTargetFieldReferenceCorrectly() {
+
+		assertThat(
+				SetOperation.builder().set("totalHomework").toValueOf("homework").toPipelineStages(contextFor(Scores.class)))
+				.containsExactly(Document.parse("{\"$set\" : {\"totalHomework\": \"$homework\" }}"));
+	}
+
+	@Test // GH-4933
+	void rendersMappedTargetFieldReferenceCorrectly() {
+
+		assertThat(SetOperation.builder().set("totalHomework").toValueOf("homework")
+				.toPipelineStages(contextFor(ScoresWithMappedField.class)))
+				.containsExactly(Document.parse("{\"$set\" : {\"totalHomework\": \"$home_work\" }}"));
+	}
+
 	@Test // DATAMONGO-2331
 	void exposesFieldsCorrectly() {