Preserve given Id on insert.

This commit fixes an issue where an existing Id got replaced with a generated one when using MongoId annotation.

Closes: #4184
Closes: #4197
Original pull request: #4203.

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

@@ -272,7 +272,7 @@ class QueryOperations {
 		 */
 		<T> MappedDocument prepareId(@Nullable MongoPersistentEntity<T> entity) {
 
-			if (entity == null) {
+			if (entity == null || source.hasId()) {
 				return source;
 			}
 

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

@@ -3636,6 +3636,26 @@ public class MongoTemplateTests {
 		assertThat(target).isEqualTo(source);
 	}
 
+	@Test // GH-4184
+	void insertHonorsExistingRawId() {
+
+		RawStringId source = new RawStringId();
+		source.id = "abc";
+		source.value = "new value";
+
+		template.insert(source);
+
+		org.bson.Document result = template
+				.execute(db -> db.getCollection(template.getCollectionName(RawStringId.class))
+						.find().limit(1).cursor().next());
+
+		assertThat(result).isNotNull();
+		assertThat(result.get("_id")).isEqualTo("abc");
+
+		RawStringId target = template.findOne(query(where("id").is(source.id)), RawStringId.class);
+		assertThat(target).isEqualTo(source);
+	}
+
 	@Test // GH-4026
 	void saveShouldGenerateNewIdOfTypeIfExplicitlyDefined() {
 

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

@@ -205,6 +205,15 @@ class QueryOperationsUnitTests {
 				});
 	}
 
+	@Test // GH-4184
+	void insertContextDoesNotOverrideExistingId() {
+
+		assertThat(queryOperations.createInsertContext(new Document("_id", "abc")).prepareId(Person.class).getDocument())//
+				.satisfies(result -> {
+					assertThat(result).isEqualTo(new Document("_id", "abc"));
+				});
+	}
+
 	static class Person {
 
 	}

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

@@ -218,6 +218,23 @@ public class ReactiveMongoTemplateTests {
 				}).verifyComplete();
 	}
 
+	@Test // GH-4184
+	void insertHonorsExistingRawId() {
+
+		MongoTemplateTests.RawStringId source = new MongoTemplateTests.RawStringId();
+		source.id = "abc";
+		source.value = "new value";
+
+		template.insert(source)
+				.then(template.execute(db -> Flux.from(
+						db.getCollection(template.getCollectionName(MongoTemplateTests.RawStringId.class)).find().limit(1).first()))
+						.next())
+				.as(StepVerifier::create).consumeNextWith(result -> {
+					assertThat(result).isNotNull();
+					assertThat(result.get("_id")).isEqualTo("abc");
+				});
+	}
+
 	@Test // DATAMONGO-1444
 	void insertsSimpleEntityCorrectly() {