Question

I have encountered strange behavoir while testing my application.
I have a request to update multiple documents and increment one of thier fields by 1 (and also decrease other by -1, but dont think that this is the point).

Query = { "_id" : { "$in" : [ObjectId("4fbf8481a975b5237c56f221"),
                             ObjectId("4fbf8481a975b5237c56f222"),
                             ObjectId("4fbf8481a975b5237c56f223")] } };
Update = { "$inc" : { "PartlyCompleted" : -1, "Completed" : 1 } }

The first document updates OK, but two others are not affected.

What could be the reason. Please give me advise where to look.

Some aditional information. I’m using C# official driver. Here is some of the code:

_personsId = new BsonArray();
_personsId.AddRange(personsId.Where(x => DataService.IsIdValid(x)).
                              Select(x => new ObjectId(x)).ToArray());

var query = Query.In("_id", _personsId);
var update = Update.Combine(
                    Update.Inc("Completed", 1 ),
                    Update.Inc("PartlyCompleted", -1));

DataService.PersonSet.Collection.Update(query, update);

I also checked the number of documents returned by the query below, and it is 3:

    var count = DataService.PersonSet.Collection.Count(query);

Answer 1

By default updates affect only the first matched document.

So, in your example mongo picks one of those 3 documents and increments counters. To update multiple documents you need to set the UpdateFlags.Multi flag.

I am curious, do you use this flag in other updates? If yes, why the question? If no, are you sure those updates touch multiple documents at once?