Response of updateMany()

[steevej (Steeve Juneau)]

@Frank_Numrich1

Is it possible that the two numbers are different, i.e. that it was not possible to update a document for some reason?

Yes. If modifiedCount is smaller than matchedCount, it means that some matched documents were not modified because they already contained the correct updated value. For example,

{ _id : 0 , a : 0 , b : 2 }
{ _id : 1 , a : 0 , b : 3 }
updateMany( { a : 0 } , { $set : { b : 2 } } )

Both documents _id:0 and _id:1 match the query a:0 so matchedCount=2 but since _id:0 already has b:2 it is not modified. Then modifiedCount=1.

I have not seen a condition where modifiedCount is > matchedCount.

@Frank_Numrich1

And how do I find out which one it is?

I do not know.