Mongodb aggregate arguments to $lookup must be strings

Question

        db.absences.insert([
           { "_id" : 1, "student" : "Ann Aardvark", sickdays: [ new Date ("2018-05-01"),new Date ("2018-08-23") ] },
           { "_id" : 2, "student" : "Zoe Zebra", sickdays: [ new Date ("2018-02-01"),new Date ("2018-05-23") ] },
        ])

    db.holidays.insert([
       { "_id" : 1, year: 2018, name: "New Years", date: new Date("2018-01-01") },
       { "_id" : 2, year: 2018, name: "Pi Day", date: new Date("2018-03-14") },
       { "_id" : 3, year: 2018, name: "Ice Cream Day", date: new Date("2018-07-15") },
       { "_id" : 4, year: 2017, name: "New Years", date: new Date("2017-01-01") },
       { "_id" : 5, year: 2017, name: "Ice Cream Day", date: new Date("2017-07-16") }
    ])

db.absences.aggregate([
   {
      $lookup:
         {
           from: "holidays",
           pipeline: [
              { $match: { year: 2018 } },
              { $project: { _id: 0, date: { name: "$name", date: "$date" } } },
              { $replaceRoot: { newRoot: "$date" } }
           ],
           as: "holidays"
         }
    }
])

I'm trying to use pipeline in lookup for aggregation query. Having this as same as from the Mongodb documentation, it still gives an error

Unable to execute the selected commands

Mongo Server error (MongoCommandException): Command failed with error 4570: 'arguments to $lookup must be strings, pipeline: [ { $match: { year: 2018.0 } }, { $project: { _id: 0.0, date: { name: "$name", date: "$date" } } }, { $replaceRoot: { newRoot: "$date" } } ] is type array' on server localhost:27017. 

The full response is:
{ 
    "ok" : 0.0, 
    "errmsg" : "arguments to $lookup must be strings, pipeline: [ { $match: { year: 2018.0 } }, { $project: { _id: 0.0, date: { name: \"$name\", date: \"$date\" } } }, { $replaceRoot: { newRoot: \"$date\" } } ] is type array", 
    "code" : NumberInt(4570), 
    "codeName" : "Location4570"
}

I'm using mongodb v3.4.

Answer 1

Because you are trying to use the $lookup features (syntax) from MongoDB v3.6 on MongoDB v3.4

The MongoDB v3.4 $lookup syntax:

{
   $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 syntax:

{
   $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/aggregation/lookup/