mongodb c driver执行mongo_run_command求sum

来源：互联网发布：牛牛网络编辑：程序博客网时间：2024/06/10 04:12

不使用mapreduce，没有必要，低，也无法取到结果。改为使用aggregate。代码如下：

 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);

最关键的是，数组的概念，如代码上面所示的数组元素0和数组元素1，这是个大坑，没有它们，就无法取得预期结果。

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