回答

可以使用mongodb的高级查询$lookup

{
   $lookup:
     {
       from: <collection to join>,
       localField: <field from the input documents>,
       foreignField: <field from the documents of the "from" collection>,
       as: <output array field>
     }
}

其实换成相当于sql语句的join用法

SELECT *, <output array field>
FROM collection
WHERE <output array field> IN (SELECT *
                               FROM <collection to join>
                               WHERE <foreignField> = <collection.localField>);

我们以php为例，来编写一个$lookup的聚合查询连表多表示例代码：

<?php
$manager = new MongoDB\Driver\Manager("mongodb://localhost:27017");

// 插入数据
$bulk = new MongoDB\Driver\BulkWrite;
// 插入商品数据
$bulk->insert(['prodid' => 1, 'title' => '产品1']);
$bulk->insert(['siteid' => 2, 'title' => '产品2']);
$_data = $manager->executeBulkWrite('test.product', $bulk);
$bulk = new MongoDB\Driver\BulkWrite;
// 插入订单数据
$bulk->insert(['orderid' => 1, 'title' => '订单1',"prodid"=>1]);
$bulk->insert(['orderid' => 2, 'title' => '订单2',"prodid"=>1]);
$_data = $manager->executeBulkWrite('test.order', $bulk);
$param = [
    'aggregate' => 'product', //表名
    'pipeline' => [
       [ '$lookup' => ['from' => "order", 'localField' => 'prodid', 'foreignField' => 'orderid', 'as' => 'orders']
    ]],

    'cursor' => new \stdClass()// 高版本需要带上这个值
];

$command = new \MongoDB\Driver\Command($param);
$cursor = $manager->executeCommand('test', $command);
foreach ($cursor as $document) {
    $document = objectToArray($document);
    var_dump($document);
}

function objectToArray($object) {
    $result = array();
    $object = is_object($object) ? get_object_vars($object) : $object;
    foreach ($object as $key => $val) {
        $val = (is_object($val) || is_array($val)) ? objectToArray($val) : $val;
        $result[$key] = $val;
    }
    return $result;
}