MongoDB UpdateOne is not updating a record

Question

I am experiencing really weird behavior when trying to update a single record. I use UpdateOne() method and it works as expected in 99% of cases BUT sometimes I get the following results: As you can see, MongoDB was able to find my record but it wasn't updated. I've tried to change write concern, which accordingly to the docs, might help:

collection.WithWriteConcern(WriteConcern.WMajority.With(journal: true))

but it didn't.

Here is how I update my records:

collection.UpdateOne(x => x.Id == _myObject.Id.AsObjectId, updateDef);

Update definition:

var updateDef = new UpdateDefinitionBuilder<IndexedProfileData>().Set(x => x.Property.ChildCollection, newCollection);

I would be really appreciated if somebody could explain me why this is happening and how to fix this behavior.

Answer 1

MongoDB will not update a document if it's already in its "updated" state.

For example, using the mongo shell:

> db.test.find()
{"_id": 0, "a": 0}

> db.test.update({_id:0}, {$set:{a:1}})
WriteResult({
  "nMatched": 1,
  "nUpserted": 0,
  "nModified": 1
})

Since a is 0 and we're setting a to 1, the update modified the document (nMatched: 1, nModified: 1).

> db.test.find()
{"_id": 0, "a": 1}

> db.test.update({_id:0}, {$set:{a:1}})
WriteResult({
  "nMatched": 1,
  "nUpserted": 0,
  "nModified": 0
})

If we're trying to set a to 1 again, the update statement found the document, but realized that it doesn't need to do any work (nMatched: 1, nModified: 0).