[{"forum":[{"msg":"Hi","readBy":["me","you"]},{"msg":"How are you?","readBy":["me"]},{"msg":"Fine, thank you","readBy":[]}]}]

db.collection.update({"forum":{"$elemMatch":{"readBy":{$ne:"me"}}}},{$push:{"forum.$[red].readBy":"me",}},{multi:true,"arrayFilters":[{"red.readBy":{$ne:"me"}}]})