处理不存在的嵌入文档的展开

Question

我正在员工表上执行聚合，以获取一些带有等投影的宿舍详细信息

$query = ['_id' => new MongoDBBSONObjectID($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);

上面的代码适用于那些存在名为"assigned_master_keys"的嵌入文档的员工，但不适用于那些不存在"assigned _master_key"的员工。分页符。如果我从上面的代码中删除展开，页面不会中断，但也不会获取宿舍数据。

请帮忙！！！

Answer 1

将$unwind与preserveNullAndEmptyArrays选项一起使用

['$unwind'=> ['path' => '$assigned_master_keys', 'preserveNullAndEmptyArrays' => true ]]