node.js - Mongoose / express : average of subdocuments

Question

我有以下型号:

产品:

var ProductSchema = new Schema({
name: String,
comments: [{ type: Schema.Types.ObjectId, ref: 'Comment'}],
_user: { type: Schema.Types.ObjectId, ref: 'User'}
});

评论:

var CommentSchema = new Schema({
text: String,
rating: Number,
_product: { type: Schema.Types.ObjectId, ref: 'Product'}
});

我目前所做的是检索所有产品及其用户:

router.get('/', function(req, res, next) {
Product.find().populate('_user').exec(function (err, products) {
    if (err) return next(err);
    res.json(products);
});
});

我想在结果中添加一个“平均”字段，其中包含每个产品的所有评论的平均值，因此结果将如下所示:

[{name: "Product 1", _user: {name: "Bob"}, average: 7.65},...]

这可以通过唯一查询实现吗？每次添加新评论时，我是否需要计算平均值并将其存储在产品文档中？

谢谢!

Answer 1

也许您应该尝试计算“运行平均值”。您只需要知道有多少评级，以及它们的平均值是多少。为 MongoDB 中的每个文档保存相同的平均值应该是一种不好的做法，希望这对您有所帮助。所以你可以像这样创建模式:

var AverageProductRatingSchema = new Schema({
  productId:       {type: Schema.Types.ObjectId, ref: 'Product'},
  averageRating:   {type: Number},
  numberOfRatings: {type: Number}
});

然后像这样实现 addRating() 函数:

function addRating(newRating, productId) {
  /* Find document that holds average rating of wanted product */
  AverageProductRating.findOneAsync({productId: productId})
    .then(function (avgProdRating) {

       /* 
         Calculate new average using the Running Average method.
         http://www.bennadel.com/blog/1627-create-a-running-average-without-storing-individual-values.htm
       */
       var newAverageRating = (avgProdRating.averageRating * avgProdRating.numberOfRatings + newRating) / (avgProdRating.numberOfRatings + 1);
       var newNumberOfRatings = avgProdRating.numberOfRatings + 1;

       AverageProductRating.update(
         { productId: productId },
         {
           $set: {
             averageRating:   newAverageRating,
             numberOfRatings: newNumberOfRatings 
         }
       });
     });

}

这是描述类似问题的链接: http://www.bennadel.com/blog/1627-create-a-running-average-without-storing-individual-values.htm