add: database functions and generalisation

src/models/relay.c

@@ -430,3 +430,14 @@ relay_get_by_controller_id(int controller_id)
     return relay_db_select(stmt);
 
 }
+
+int
+relay_get_controller_id_for_relay(int relay_id)
+{
+    sqlite3_stmt *stmt;
+
+    sqlite3_prepare_v2(global_database, "SELECT controller_id FROM relays WHERE relay_id=?1;", -1, &stmt, NULL);
+    sqlite3_bind_int(stmt, 1, relay_id);
+
+    return database_helper_get_id(stmt);
+}