Add support for keypaths crossing backlins

src/realm/object-store/c_api/notifications.cpp

@@ -73,7 +73,7 @@ std::optional<KeyPathArray> build_key_path_array(realm_key_path_array_t* key_pat
     return ret;
 }
 
-static KeyPathArray create_key_path_array(const ObjectSchema& object_schema, const Schema& schema,
+static KeyPathArray create_key_path_array(const Group& group, const ObjectSchema& object_schema, const Schema& schema,
                                           const char** all_key_paths_begin, const char** all_key_paths_end)
 {
     KeyPathArray resolved_key_path_array;
@@ -99,7 +99,20 @@ static KeyPathArray create_key_path_array(const ObjectSchema& object_schema, con
             }
             prop = schema_at_index->property_for_public_name(property);
             if (prop) {
-                resolved_key_path.emplace_back(schema_at_index->table_key, prop->column_key);
+                if ((to_underlying(prop->type & PropertyType::LinkingObjects)) != 0) {
+                    // TODO Why isn't this just filled in the PropertyInfo::column_key?
+                    auto originClass = schema.find(prop->object_type);
+                    if (originClass != schema.end()) {
+                        ColKey forwardColKey = originClass->property_for_name(prop->link_origin_property_name)->column_key;
+                        ConstTableRef originTable = group.get_table(originClass->table_key);
+                        resolved_key_path.emplace_back(schema_at_index->table_key, originTable->get_opposite_column(forwardColKey));
+                    } else {
+                        throw InvalidArgument(util::format("Property '%1' in Keypath '%2' is a backlink that points to a class that could not be found: %3",
+                                              property, *it, prop->object_type));
+                    }
+                } else {
+                    resolved_key_path.emplace_back(schema_at_index->table_key, prop->column_key);
+                }
                 schema_at_index = nullptr;
             }
             else {
@@ -121,9 +134,10 @@ RLM_API realm_key_path_array_t* realm_create_key_path_array(const realm_t* realm
     return wrap_err([&]() {
         KeyPathArray ret;
         if (user_key_paths) {
+            const Group& group = (*realm)->read_group();
             const Schema& schema = (*realm)->schema();
             const ObjectSchema& object_schema = schema_for_table(*realm, TableKey(object_class_key));
-            ret = create_key_path_array(object_schema, schema, user_key_paths, user_key_paths + user_key_paths_count);
+            ret = create_key_path_array(group, object_schema, schema, user_key_paths, user_key_paths + user_key_paths_count);
         }
         return new realm_key_path_array_t(std::move(ret));
     });