如何对Firebase Firestore中的节点求和? [英] How to Sum nodes in firebase firestore?

问题描述

我正在制作用于订购比萨饼的应用程序.

I am making app for ordering pizza.

就像您在图片上看到的那样，我想从每个文档中获取所有名为"ukupno"的键值，并将其求和为最终价格.

Like you see on image i want to take all key values named "ukupno" from each document and Sum it for final price.

这是我获取购物车数据的方式:

This is how i fetch cart data:

  this.cartService.getCart(this.user.Uid).subscribe(res => {
      this.cart = res.map(i => {
        return {
          ...i.payload.doc.data(),
          id: i.payload.doc.id
        };
      });
    });
  }
 getCart(uid) {
    return this.firestore
      .collection("cart/")
      .doc(uid)
      .collection("/ubaceno")
      .snapshotChanges();
  }

推荐答案

对于集合的所有文档，基本上有两种方法可以汇总文档字段的值:

There are basically two ways to sum up the value of a document field for all documents of a collection:

1/查询整个集合，遍历结果(即 snapshotChanges() .

1/ Query for the entire collection, loop over the result (i.e. the querySnapshot), e.g. with forEach(), and sum all the field values in the loop. To do that with angularfire2 you would probably use snapshotChanges().

但是，如果您的馆藏中包含很多文档，这不是很有效，而且在您的馆藏中每个文档需要花费一个文档读取.

However, If your collection contains a lot of documents, this is not very efficient and moreover it will cost one document read per document in your collection.

2/使用一些分布式计数器 保持所有文档的最新金额.要实现此方法，您每次在 ubaceno 集合中创建/修改/删除文档时都必须更新计数器，将 ukupno 字段的值添加或减去/从柜台.

2/ Use some Distributed counters to keep an up-to-date sum for all your documents. To implement this approach, you have to update the counter each time you create/modify/delete a document in the ubaceno collection, adding or subtracting the value of the ukupno field to/from the counter.

您可以通过具有一组在创建/修改/删除时更新计数器的云函数，或者通过在前端的写入/修改/删除代码中集成计数器更新的事务来做到这一点.

You could do that either by having a set of Cloud Functions that update the counters upon Creation/Modification/Deletion, or by integrating in your write/modify/delete code in your front end the transaction for the counter update.

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