Fix basic update overriding values.

This change makes sure basic update appends values to operations instead of overriding them. This change aligns the behaviour with Update and fixes issues where using the Update annotation with versioned entities can lead to loss of update information.

Closes: #4918
Original pull request: #4921

Author: christophstrobl

Diff for: spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/query/BasicUpdate.java

@@ -15,11 +15,14 @@
  */
 package org.springframework.data.mongodb.core.query;
 
-import java.util.Arrays;
 import java.util.Collections;
+import java.util.LinkedHashMap;
+import java.util.List;
+import java.util.Map;
 
 import org.bson.Document;
 import org.springframework.lang.Nullable;
+import org.springframework.util.ClassUtils;
 
 /**
  * @author Thomas Risberg
@@ -44,63 +47,85 @@ public BasicUpdate(Document updateObject) {
 
 	@Override
 	public Update set(String key, @Nullable Object value) {
-		updateObject.put("$set", Collections.singletonMap(key, value));
+		setOperationValue("$set", key, value);
 		return this;
 	}
 
 	@Override
 	public Update unset(String key) {
-		updateObject.put("$unset", Collections.singletonMap(key, 1));
+		setOperationValue("$unset", key, 1);
 		return this;
 	}
 
 	@Override
 	public Update inc(String key, Number inc) {
-		updateObject.put("$inc", Collections.singletonMap(key, inc));
+		setOperationValue("$inc", key, inc);
 		return this;
 	}
 
 	@Override
 	public Update push(String key, @Nullable Object value) {
-		updateObject.put("$push", Collections.singletonMap(key, value));
+		setOperationValue("$push", key, value);
 		return this;
 	}
 
 	@Override
 	public Update addToSet(String key, @Nullable Object value) {
-		updateObject.put("$addToSet", Collections.singletonMap(key, value));
+		setOperationValue("$addToSet", key, value);
 		return this;
 	}
 
 	@Override
 	public Update pop(String key, Position pos) {
-		updateObject.put("$pop", Collections.singletonMap(key, (pos == Position.FIRST ? -1 : 1)));
+		setOperationValue("$pop", key, (pos == Position.FIRST ? -1 : 1));
 		return this;
 	}
 
 	@Override
 	public Update pull(String key, @Nullable Object value) {
-		updateObject.put("$pull", Collections.singletonMap(key, value));
+		setOperationValue("$pull", key, value);
 		return this;
 	}
 
 	@Override
 	public Update pullAll(String key, Object[] values) {
-		Document keyValue = new Document();
-		keyValue.put(key, Arrays.copyOf(values, values.length));
-		updateObject.put("$pullAll", keyValue);
+		setOperationValue("$pullAll", key, List.of(values));
 		return this;
 	}
 
 	@Override
 	public Update rename(String oldName, String newName) {
-		updateObject.put("$rename", Collections.singletonMap(oldName, newName));
+		setOperationValue("$rename", oldName, newName);
 		return this;
 	}
 
+	@Override
+	public boolean modifies(String key) {
+		return super.modifies(key) || Update.fromDocument(getUpdateObject()).modifies(key);
+	}
+
 	@Override
 	public Document getUpdateObject() {
 		return updateObject;
 	}
 
+	void setOperationValue(String operator, String key, Object value) {
+
+		if (!updateObject.containsKey(operator)) {
+			updateObject.put(operator, Collections.singletonMap(key, value));
+		} else {
+			Object existingValue = updateObject.get(operator);
+			if (existingValue instanceof Map<?, ?> existing) {
+				Map<Object, Object> target = new LinkedHashMap<>(existing);
+				target.put(key, value);
+				updateObject.put(operator, target);
+			} else {
+				throw new IllegalStateException(
+						"Cannot add ['%s' : { '%s' : ... }]. Operator already exists with value of type [%s] which is not suitable for appending"
+								.formatted(operator, key,
+										existingValue != null ? ClassUtils.getShortName(existingValue.getClass()) : "null"));
+			}
+		}
+	}
+
 }

Diff for: spring-data-mongodb/src/test/java/org/springframework/data/mongodb/core/MongoTemplateUpdateTests.java

@@ -34,6 +34,7 @@
 import org.springframework.data.mongodb.core.aggregation.SetOperation;
 import org.springframework.data.mongodb.core.mapping.Document;
 import org.springframework.data.mongodb.core.mapping.Field;
+import org.springframework.data.mongodb.core.query.BasicUpdate;
 import org.springframework.data.mongodb.core.query.Criteria;
 import org.springframework.data.mongodb.core.query.Query;
 import org.springframework.data.mongodb.core.query.Update;
@@ -289,6 +290,20 @@ void nullValueShouldBePropagatedToDatabase() {
 				null);
 	}
 
+	@Test // GH-4918
+	void updateShouldHonorVersionProvided() {
+
+		Versioned source = template.insert(Versioned.class).one(new Versioned("id-1", "value-0"));
+
+		Update update = new BasicUpdate("{ '$set' : { 'value' : 'changed' }, '$inc' : { 'version' : 10 } }");
+		template.update(Versioned.class).matching(Query.query(Criteria.where("id").is(source.id))).apply(update).first();
+
+		assertThat(
+			collection(Versioned.class).find(new org.bson.Document("_id", source.id)).limit(1).into(new ArrayList<>()))
+			.containsExactly(new org.bson.Document("_id", source.id).append("version", 10L).append("value", "changed")
+				.append("_class", "org.springframework.data.mongodb.core.MongoTemplateUpdateTests$Versioned"));
+	}
+
 	private List<org.bson.Document> all(Class<?> type) {
 		return collection(type).find(new org.bson.Document()).into(new ArrayList<>());
 	}

Diff for: spring-data-mongodb/src/test/java/org/springframework/data/mongodb/core/query/BasicUpdateUnitTests.java

@@ -0,0 +1,97 @@
+/*
+ * Copyright 2025 the original author or authors.
+ *
+ * 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 org.springframework.data.mongodb.core.query;
+
+import static org.assertj.core.api.Assertions.assertThatExceptionOfType;
+import static org.springframework.data.mongodb.test.util.Assertions.assertThat;
+
+import java.util.function.Function;
+import java.util.stream.Stream;
+
+import org.bson.Document;
+import org.junit.jupiter.api.Test;
+import org.junit.jupiter.params.ParameterizedTest;
+import org.junit.jupiter.params.provider.Arguments;
+import org.junit.jupiter.params.provider.CsvSource;
+import org.junit.jupiter.params.provider.MethodSource;
+import org.springframework.data.mongodb.core.query.Update.Position;
+
+/**
+ * @author Christoph Strobl
+ */
+public class BasicUpdateUnitTests {
+
+	@Test // GH-4918
+	void setOperationValueShouldAppendsOpsCorrectly() {
+
+		BasicUpdate basicUpdate = new BasicUpdate("{}");
+		basicUpdate.setOperationValue("$set", "key1", "alt");
+		basicUpdate.setOperationValue("$set", "key2", "nps");
+		basicUpdate.setOperationValue("$unset", "key3", "x");
+
+		assertThat(basicUpdate.getUpdateObject())
+				.isEqualTo("{ '$set' : { 'key1' : 'alt', 'key2' : 'nps' }, '$unset' : { 'key3' : 'x' } }");
+	}
+
+	@Test // GH-4918
+	void setOperationErrorsOnNonMapType() {
+
+		BasicUpdate basicUpdate = new BasicUpdate("{ '$set' : 1 }");
+		assertThatExceptionOfType(IllegalStateException.class)
+				.isThrownBy(() -> basicUpdate.setOperationValue("$set", "k", "v"));
+	}
+
+	@ParameterizedTest // GH-4918
+	@CsvSource({ //
+			"{ }, k1, false", //
+			"{ '$set' : { 'k1' : 'v1' } }, k1, true", //
+			"{ '$set' : { 'k1' : 'v1' } }, k2, false", //
+			"{ '$set' : { 'k1.k2' : 'v1' } }, k1, false", //
+			"{ '$set' : { 'k1.k2' : 'v1' } }, k1.k2, true", //
+			"{ '$set' : { 'k1' : 'v1' } }, '', false", //
+			"{ '$inc' : { 'k1' : 1 } }, k1, true" })
+	void modifiesLooksUpKeyCorrectly(String source, String key, boolean modified) {
+
+		BasicUpdate basicUpdate = new BasicUpdate(source);
+		assertThat(basicUpdate.modifies(key)).isEqualTo(modified);
+	}
+
+	@ParameterizedTest // GH-4918
+	@MethodSource("updateOpArgs")
+	void updateOpsShouldNotOverrideExistingValues(String operator, Function<BasicUpdate, Update> updateFunction) {
+
+		Document source = Document.parse("{ '%s' : { 'key-1' : 'value-1' } }".formatted(operator));
+		Update update = updateFunction.apply(new BasicUpdate(source));
+
+		assertThat(update.getUpdateObject()).containsEntry("%s.key-1".formatted(operator), "value-1")
+				.containsKey("%s.key-2".formatted(operator));
+	}
+
+	static Stream<Arguments> updateOpArgs() {
+
+		return Stream.of( //
+				Arguments.of("$set", (Function<BasicUpdate, Update>) update -> update.set("key-2", "value-2")),
+				Arguments.of("$unset", (Function<BasicUpdate, Update>) update -> update.unset("key-2")),
+				Arguments.of("$inc", (Function<BasicUpdate, Update>) update -> update.inc("key-2", 1)),
+				Arguments.of("$push", (Function<BasicUpdate, Update>) update -> update.push("key-2", "value-2")),
+				Arguments.of("$addToSet", (Function<BasicUpdate, Update>) update -> update.addToSet("key-2", "value-2")),
+				Arguments.of("$pop", (Function<BasicUpdate, Update>) update -> update.pop("key-2", Position.FIRST)),
+				Arguments.of("$pull", (Function<BasicUpdate, Update>) update -> update.pull("key-2", "value-2")),
+				Arguments.of("$pullAll",
+						(Function<BasicUpdate, Update>) update -> update.pullAll("key-2", new String[] { "value-2" })),
+				Arguments.of("$rename", (Function<BasicUpdate, Update>) update -> update.rename("key-2", "value-2")));
+	};
+}