mongocxx c++ 14标准，进行多表联合查询

#include <mongocxx/client.hpp>
#include <mongocxx/instance.hpp>
#include <mongocxx/uri.hpp>
#include <bsoncxx/builder/stream/document.hpp>
#include <bsoncxx/json.hpp>
#include <bsoncxx/types.hpp>

using bsoncxx::builder::stream::document;
using bsoncxx::builder::stream::finalize;
using bsoncxx::builder::stream::open_array;
using bsoncxx::builder::stream::close_array;
using bsoncxx::builder::stream::open_document;
using bsoncxx::builder::stream::close_document;
using bsoncxx::builder::stream::concatenate;
using bsoncxx::builder::basic::make_document;
using bsoncxx::builder::basic::kvp;

    
mongocxx::instance inst{}; // 实例化 Mongocxx 库
    mongocxx::uri uri("mongodb://localhost:27017"); // 连接 MongoDB URI
    mongocxx::client conn(uri); // 创建连接实例

    mongocxx::database db = conn["test_sh"];

    mongocxx::collection teacher_1 = db["teacher_20230417_0_60"];
    mongocxx::collection teacher_2 = db["teacher_20230417_120_180"];
    // mongocxx::collection teacher_3 = db["teacher_20230417_180_240"];

    auto pipeline = mongocxx::pipeline{};
    pipeline.lookup({
        make_document(
            kvp("from", "teacher_20230417_120_180"), // b表的表名
            kvp("localField", "teacher_1.index_join_id"), // a表的关联外键
            kvp("foreignField", "teacher_2.index_join_id"), // b表的关联外键
            kvp("as", "teacher_2") // 别名
        )
        });


    pipeline.unwind("$teacher_2");
    // 如果有多个，这里需要写多个
    /*pipeline.unwind("$array_field1");
    pipeline.unwind("$array_field2");
    pipeline.unwind("$array_field3");*/


    // 这里是做展示代码
    pipeline.project({
        make_document(
            kvp("_id", 0),
            kvp("index", 1),
            kvp("time", 1),
            kvp("TPE02", "$teacher_2.TPE02")
        )
        });

    // 排序
    // pipeline.sort(make_document(kvp("time", 1)));

    pipeline.limit(10); // 使用.limit()操作符限制结果数量


    auto cursor = teacher_1.aggregate(pipeline);
    
    for (auto&& doc : cursor) {
        std::cout << bsoncxx::to_json(doc) << std::endl;
    }