Why mongodb upsert counter doesn't reflect actual changes? - mongodb

Question

Consider following query
var collectionName = "test";
db.createCollection(collectionName);
db.getCollection(collectionName).insert({
"_id" : 1, "char" : "Gandalf", "class" : "barbarian", "lvl" : 20
});
db.getCollection(collectionName).bulkWrite([
{
insertOne: {
"document": {
"_id" : 2, "char" : "Dithras", "class" : "barbarian", "lvl" : 4
}
}
},
{
updateOne: {
filter: { "_id": 1},
update: { $set: {"class" : "mage"} },
upsert: true
}
}
])
Which results in:
{
"acknowledged" : true,
"deletedCount" : 0.0,
"insertedCount" : 1.0,
"matchedCount" : 1.0,
"upsertedCount" : 0.0,
"insertedIds" : {
"0" : 2.0
},
"upsertedIds" : {}
}
my question is why updating of a document with id:1 doesn't get into upsertedIds? Isn't this document just being updated with upsert? Or am I missing anything?
According to the documentation it only adds information to upsert if it doesn't find any document (so it's actually more like inserted), but this case I don't know which items got updated.
Is it possible to get which documents got modified when executing a query?
To avoid XY problem: I want to see bulk operation items taht failed (e.g. when trying to update non-existing document with upsert:false) and log IDs that triggered the failures.

Answer 1

You can collect document ids when you prepare your bulk update, then check which of them don't exist after the update. E.g. for document ids 1, 2, and 3 that you updated with upsert:false
db.test.aggregate([
{ $group: { _id: null }},
{ $project: { _id: [ 1, 2, 3 ] }},
{ $unwind: "$_id" },
{ $lookup: {
from: "test",
localField: "_id",
foreignField: "_id",
as: "exists"
}},
{ $match: { exists: { $size:0 }}},
{ $project: { exists:0 }}
])
will return id's of filters that didn't match any document, i.e. "failed" in your terminology.