Mongoose retun nModified : 1 when no data is updated - mongodb

Question

Trying to understand why/if mongoose is updating my documents even though no data is changed?
If I save a new document with the query below it will return this in the console.log(item)
{ n: 1,
nModified: 0,
upserted: [ { index: 0, _id: 5f3d35c386aeb6c6fb35fa79 } ],
ok: 1 }
Query
Product.updateOne(
{productName: product.productName},
{$set: newProduct},
{upsert: true}
).then((item) => {
console.log(item);
}).catch((e) => {
console.log('Insert error', e);
});
If i rerun the same query again i get this back. This indicates that the document has been modified but the data is the same, there is no new data that has been inserted.
{ n: 1, nModified: 1, ok: 1 }
I've noticed if i remove the stores array, delete the document, insert it again and rerun the query I get { n: 1, nModified: 0, ok: 1 } back in the console.log(item)
I run the same querys, the same amout of time, but when having an array in the object i get this { n: 1, nModified: 1, ok: 1 } and when not having an array a get this { n: 1, nModified: 0, ok: 1 }
It seems that when having an array the document gets modified regardless if the data is changed.
Example 1
Gives { n: 1, nModified: 1, ok: 1 }
const newProduct = {
ean: product.ean,
productName: product.productName,
lowestPrice: product.productPrice,
mainCategory: categories.mainCategory,
group: categories.group,
subCategory: categories.subCategory,
subSubCategory: subSubCat,
stores: [{
name: "foobar",
}],
};
Example 2
Gives { n: 1, nModified: 0, ok: 1 }
const newProduct = {
ean: product.ean,
productName: product.productName,
lowestPrice: product.productPrice,
mainCategory: categories.mainCategory,
group: categories.group,
subCategory: categories.subCategory,
subSubCategory: subSubCat,
};
Is it me who misunderstands the operation below or whats going on?
What i want to do is:
1.insert if the document don't exists based on productName,
2.if something differs in the document stored in the database and the newProduct, update the document.
3.If nothing differs, do nothing
Product model
const ProductSchema = new Schema({
ean: String,
productName: String,
mainCategory: String,
subCategory: String,
group: String,
subSubCategory: String,
lowestPrice: Number,
isPopular: Boolean,
description: String,
stores: [
{
name: String,
},
],
});
Edit: As its pretty hard to explain I created a small repo that shows the issue.
https://github.com/gameatrix/mongo_array

Answer 1

The database still performs the update.
For example, let's conditionally upsert a value:
MongoDB Enterprise ruby-driver-rs:PRIMARY> db.foo.update({a:42},{a:42},{upsert:true})
WriteResult({
"nMatched" : 0,
"nUpserted" : 1,
"nModified" : 0,
"_id" : ObjectId("5f3d4ed509fcd40c9f092690")
})
MongoDB Enterprise ruby-driver-rs:PRIMARY> db.foo.update({a:42},{a:42},{upsert:true})
WriteResult({ "nMatched" : 1, "nUpserted" : 0, "nModified" : 1 })
The first write was an insert, the second write was an update. The second write did not change any data but the database performed a write.
You can verify there was a write by using a change stream in another shell instance:
MongoDB Enterprise ruby-driver-rs:PRIMARY> db.foo.watch()
{ "_id" : { "_data" : "825F3D4ED5000000012B022C0100296E5A1004FAC29486D5A3459A8726349007F2E43E46645F696400645F3D4ED509FCD40C9F0926900004" }, "operationType" : "insert", "clusterTime" : Timestamp(1597853397, 1), "fullDocument" : { "_id" : ObjectId("5f3d4ed509fcd40c9f092690"), "a" : 42 }, "ns" : { "db" : "test", "coll" : "foo" }, "documentKey" : { "_id" : ObjectId("5f3d4ed509fcd40c9f092690") } }
{ "_id" : { "_data" : "825F3D4ED6000000012B022C0100296E5A1004FAC29486D5A3459A8726349007F2E43E46645F696400645F3D4ED509FCD40C9F0926900004" }, "operationType" : "replace", "clusterTime" : Timestamp(1597853398, 1), "fullDocument" : { "_id" : ObjectId("5f3d4ed509fcd40c9f092690"), "a" : 42 }, "ns" : { "db" : "test", "coll" : "foo" }, "documentKey" : { "_id" : ObjectId("5f3d4ed509fcd40c9f092690") } }
By definition an upsert either modifies documents that match the condition or inserts new documents. You are always going to have a write when upserting.
2.if something differs in the document stored in the database and the newProduct, update the document.
The bolded part is not how MongoDB (and most databases, as far as I know) work. Whether a write is performed does not depend on whether the data being written is the same as what is already in the database.