DATAMONGO-2043 - Omit type hint when mapping simple types.

Original pull request: #589.

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

@@ -412,12 +412,23 @@ public class MappingMongoConverter extends AbstractMongoConverter implements App
 			removeFromMap(bson, "_id");
 		}
 
-		boolean handledByCustomConverter = conversions.hasCustomWriteTarget(entityType, Document.class);
-		if (!handledByCustomConverter && !(bson instanceof Collection)) {
+		if (requiresTypeHint(entityType)) {
 			typeMapper.writeType(type, bson);
 		}
 	}
 
+	/**
+	 * Check if a given type requires a type hint {@literal aka _class attribute} when writing to the document.
+	 *
+	 * @param type must not be {@literal null}.
+	 * @return true if not a simple type, collection or type with custom write target.
+	 */
+	private boolean requiresTypeHint(Class<?> type) {
+
+		return !conversions.isSimpleType(type) && !ClassUtils.isAssignable(Collection.class, type)
+				&& !conversions.hasCustomWriteTarget(type, Document.class);
+	}
+
 	/**
 	 * Internal write conversion method which should be used for nested invocations.
 	 *

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

@@ -1904,6 +1904,15 @@ public class MappingMongoConverterUnitTests {
 		assertThat(converter.read(Attribute.class, source).value).isInstanceOf(List.class);
 	}
 
+	@Test // DATAMONGO-2043
+	public void omitsTypeHintWhenWritingSimpleTypes() {
+
+		org.bson.Document target = new org.bson.Document();
+		converter.write(new org.bson.Document("value", "FitzChivalry"), target);
+
+		assertThat(target).doesNotContainKeys("_class");
+	}
+
 	static class GenericType<T> {
 		T content;
 	}