处理不存在的嵌入文档的展开 [英] Handling unwind for the non existing embedded document
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问题描述
我正在员工表上执行聚合以获取一些带有投影的宿舍详细信息
I am performing aggregation on employees table to fetch some hostel details with projection like
$query = ['_id' => new MongoDBBSONObjectID($this->EmployeeId)];
$pipeline = array(
['$match' => $query],
[ '$addFields'=> [
'assigned_master_keys'=> [
'$cond'=> [
'if'=> [
'$ne'=> [ [ '$type'=> '$assigned_master_keys' ], 'array' ]
],
'then'=> [],
'else'=> '$assigned_master_keys'
]
]
]],
['$unwind'=> '$assigned_master_keys'],
['$lookup' => [
'from' => 'HostelTbl',
'let' => [ 'hostelid' => '$assigned_master_keys.hostel_id' ],
'pipeline' => [
['$match' => [ '$expr'=> [ '$eq' => [ '$_id', '$$hostelid' ]]]],
['$project' => [ 'Name'=> 1, 'MasterKeyDetails' => 1, '_id'=> 1]]
],
'as' => 'assigned_master_keys.hostel_id'
]],
);
$this->collection = $this->db->EmployeesTbl;
$cursor = $this->collection->aggregate($pipeline);
上述代码适用于存在名称为assigned_master_keys"的嵌入文档的员工,但不适用于不存在assigned_master_keys"的员工.分页符.如果我从上面的代码中删除 unwind,页面不会中断,但也不会获取旅馆数据.
The above code works fine for those employees where embedded document with name "assigned_master_keys" exists but not for those where "assigned_master_keys" does not exists. The page breaks. If I remove unwind from the above code, the page does not break but it does not fetch hostel data either.
请帮忙!!!
推荐答案
使用 $unwind
和 preserveNullAndEmptyArrays
选项
['$unwind'=> ['path' => '$assigned_master_keys', 'preserveNullAndEmptyArrays' => true ]]
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