mongodb c driver执行mongo_run_command求sum

sunyke

　　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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