Agregační argumenty Mongodb pro $lookup musí být řetězce

Protože se pokoušíte použít $lookup funkce (syntaxe) z MongoDB v3.6 na MongoDB v3.4

The MongoDB v3.4 $lookup syntaxe:

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

The MongoDB v3.6 $lookup syntaxe:

{
   $lookup:
     {
       from: <collection to join>,
       let: { <var_1>: <expression>, …, <var_n>: <expression> },
       pipeline: [ <pipeline to execute on the collection to join> ],
       as: <output array field>
     }
}

https://docs.mongodb.com/manual/reference/operator/ agregace/vyhledávání/