c# - Mongo update response says no document updated, but the document is there -

i'm using mongodb 2.6.10 , using c# driver 1.9.2. server has replicaset of 2 servers.

my documents of format. itemid unique.

{     "itemid": 2314,     "favorites": [1, 24, 26, 34] } 

then have code remove favorite of form

var query = query.eq("itemid", itemid); var result = collection.update(query, update.pull("favorites", favoriteidtoremove)); 

after each time, check result.documentsaffected equal 1. once in while, value comes 0. when go mongodb myself, can find document matching itemid, , can see favoriteid tried remove in array still there. result.ok true, , there's no error information.

what cause fail?

i'm no expert guess write concerns since there different levels of guarantee writing , updating documents in mongodb. see write concerns

so instead of using method:

mongocollection.update method (imongoquery, imongoupdate) 

it might better use method instead:

mongocollection.update method (imongoquery, imongoupdate, writeconcern) 

and specify writeconcern be

writeconcern.wmajority 

that way update has highest guarantee.