[{"price":null,"model":[{"price_option":"200","currency_unit":"dollar","stock_count":5},{"price_option":"350","currency_unit":"dollar","stock_count":0},{"price_option":"400","currency_unit":"dollar","stock_count":2},]}]

db.collection.aggregate([{$set:{price_model:{$arrayElemAt:[{"$filter":{"input":"$model","cond":{$gt:["$$this.stock_count",0]}}},0]}}},{$project:{_id:0,id:"$_id",price:"$price_model.price_option",model:1}}])