Add test to verify map keys retain order when mapped.

See: #4577
Original pull request: #4568

Author: christophstrobl

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

@@ -28,6 +28,7 @@
 import java.util.List;
 import java.util.Map;
 import java.util.Optional;
+import java.util.TreeMap;
 
 import org.bson.conversions.Bson;
 import org.bson.types.Code;
@@ -1509,6 +1510,20 @@ void convertsListOfValuesForPropertyThatHasValueConverterButIsNotCollectionLikeO
 		assertThat(mappedObject).isEqualTo("{ 'text' : { $in : ['gnirps', 'atad'] } }");
 	}
 
+	@Test // GH-4577
+	void mappingShouldRetainMapKeyOrder() {
+
+		TreeMap<String, String> sourceMap = new TreeMap<>(Map.of("test1", "123", "test2", "456"));
+
+		org.bson.Document target = mapper.getMappedObject(query(where("simpleMap").is(sourceMap)).getQueryObject(),
+				context.getPersistentEntity(WithSimpleMap.class));
+		assertThat(target.get("simpleMap", Map.class)).containsExactlyEntriesOf(sourceMap);
+	}
+
+	class WithSimpleMap {
+		Map<String, String> simpleMap;
+	}
+
 	class WithDeepArrayNesting {
 
 		List<WithNestedArray> level0;