[{"_id":ObjectId("573a1390f29313caabcd4cf1"),"title":"Ingeborg Holm","cast":["Aron Lindgren","Erik Lindholm",],"imdb":{"rating":7,},},{"_id":ObjectId("573a1390f29313caabcd4cf3"),"title":"Ingeborg Holm","cast":["Aron Lindgren","Erik Lindholm",],"imdb":{"rating":"",},},{"_id":ObjectId("573a1390f29313caabcd4cf4"),"title":"Ingeborg Holm","cast":["Aron Lindgren","Erik Lindholm",],"imdb":{"rating":null,},}]

db.collection.aggregate([{$unwind:{path:"$cast"}},{$set:{"average":0}},{$group:{_id:"$cast",numFilms:{$sum:1},average:{$avg:{$add:["$average",{$convert:{input:"$imdb.rating",to:"int",onError:0,onNull:0}}]},/**failed here*/}}}])