mongodb c driver执行mongo_run_command求sum

bson cmd[1];
bson out[1];
bson_init(cmd);
bson_append_string(cmd, "aggregate", "mytable");

bson_append_start_array(cmd, "pipeline");
bson_append_start_object(cmd, "0");
bson_append_start_object(cmd, "$group");
bson_append_null(cmd, "_id");
//bson_append_string(cmd, "_id", "null");
bson_append_start_object(cmd, "sum");
bson_append_string(cmd, "$sum", "$a_price");
bson_append_finish_object(cmd);
bson_append_finish_object(cmd);
bson_append_finish_object(cmd);

bson_append_start_object(cmd, "1");
bson_append_start_object(cmd, "$project");
bson_append_int(cmd, "_id", 0);
bson_append_int(cmd, "sum", 1);
bson_append_finish_object(cmd);
bson_append_finish_object(cmd);

bson_append_finish_array(cmd);

bson_finish(cmd);
bson_print(cmd);

const int result = mongo_run_command(conn, "mydb", cmd, out);