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\") ] },
                  


        

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/

Answer 2

In $lookup you could just join the collectionfirst and doing the match, project and etc later after lookup. And you should have the foreign field that referring to holidays collection.

Like :

{
  $lookup:
     {
       from: "holidays",
       localField: "holidaysID", //yourLocalfield, it is local field in absences. you should have it to join 2 collections
       foreignField : "_id", //theforeignfield, it is _id in the holidays
       as: "holidays"
     }
},{ $match: { year: 2018 } },
{ $project: { _id: 0, date: { name: "$name", date: "$date" } } },
{ $replaceRoot: { newRoot: "$date" } }

Hope it helps.