Adjust replaceWith type pipeline stage

[FabioCingottini]

Summary

For how the $replaceWith pipeline stage type was written, it wasn't possible to replace the document by using expressions.

Examples

Things like the following were not possible, now they're.

db.sales.insertMany([
  { "_id" : 1, "item" : "butter", "price" : 10, "quantity": 2, date: ISODate("2019-03-01T08:00:00Z"), status: "C" },
  { "_id" : 2, "item" : "cream", "price" : 20, "quantity": 1, date: ISODate("2019-03-01T09:00:00Z"), status: "A" },
  { "_id" : 3, "item" : "jam", "price" : 5, "quantity": 10, date: ISODate("2019-03-15T09:00:00Z"), status: "C" },
  { "_id" : 4, "item" : "muffins", "price" : 5, "quantity": 10, date: ISODate("2019-03-15T09:00:00Z"), status: "C" }
])

SalesModel.aggregate([
    { $match: { status: "C" } },
    { 
        $replaceWith: {
            _id: "$_id",
            item: "$item",
            amount: {
                $multiply: [ "$price", "$quantity"]
            },
            status: "Complete", asofDate: "$NOW" 
        }
    }
])

[AbdelrahmanHafez]

Can you please add tests to ./test/types to avoid a future regression?
Thanks @FabioCingottini

[vkarpov15]

Thanks 👍

[feelgood-interface]

Shouldn't string be allowed too?

db.collection.aggregate([
{ $replaceWith: "$name" }
])

[vkarpov15]

@feelgood-interface that was added in 616ce5d