Fix meta field mapping when computing fields for projections.

Related to: #3894

Author: christophstrobl

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

@@ -302,6 +302,7 @@ Document getMappedFields(@Nullable MongoPersistentEntity<?> entity,
 				mappedFields = queryMapper.getMappedFields(fields, entity);
 			} else {
 				mappedFields = propertyOperations.computeMappedFieldsForProjection(projection, fields);
+				mappedFields = queryMapper.addMetaAttributes(mappedFields, entity);
 			}
 
 			if (entity.hasTextScoreProperty() && mappedFields.containsKey(entity.getTextScoreProperty().getFieldName())

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

@@ -240,6 +240,18 @@ private Document mapFieldsToPropertyNames(Document fields, @Nullable MongoPersis
 		return target;
 	}
 
+	/**
+	 * Adds missing {@code $meta} representation if required.
+	 *
+	 * @param source must not be {@literal null}.
+	 * @param entity can be {@literal null}.
+	 * @return never {@literal null}.
+	 * @since 3.4
+	 */
+	public Document addMetaAttributes(Document source, @Nullable MongoPersistentEntity<?> entity) {
+		return mapMetaAttributes(source, entity, MetaMapping.FORCE);
+	}
+
 	private Document mapMetaAttributes(Document source, @Nullable MongoPersistentEntity<?> entity,
 			MetaMapping metaMapping) {