如何在firebase firestore中求和节点? [英] How to Sum nodes in firebase firestore?

问题描述

我正在制作用于订购披萨的应用.

I am making app for ordering pizza.

就像你在图片上看到的那样，我想从每个文档中取出所有名为ukupno"的键值，然后将其求和以获得最终价格.

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

对集合的所有文档求和一个文档字段的值，基本上有两种方法:

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 querySnapshot)，例如使用 forEach()，并对循环中的所有字段值求和.要使用 angularfire2 做到这一点，您可能会使用 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，如果您的集合包含大量文档，这不是很有效，而且您的集合中的每个文档需要读取一个文档.

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件的最新金额.要实现这种方法，您必须在每次创建/修改/删除 ubaceno 集合中的文档时更新计数器，添加或减去 ukupno 字段的值以/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.

您可以通过使用一组在创建/修改/删除时更新计数器的云函数来做到这一点，或者通过在前端的写入/修改/删除代码中集成计数器更新事务.

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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