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