如何从文档中获取特定字段值

Question

I'm trying to get values from documents in collection using Firestore.

This is my code and console log:

 useEffect(() => {
    let totalarray = [];
   firebase.firestore().collection("users").doc(uid).collection("services").get().then((snapshot) => {
     snapshot.forEach((doc) => {
       const obj = doc.data();
       //obj.forEach((data) => totalarray.push((data)) )
      console.log("------->",obj);
     })
   })
  }, [])

This is my Console Log:

My Goal is to take the value of each "serviceCost" field for each doc, sum it up and then put it in a global variable for my use.

Answer 1

If you mean that you would like to get just some parts of Firestore documents then you cannot. you have to read the whole document then do your calculations client side. other method, you may consider creating another collection like "totals". this collection will be populated by Firebase functions that invokes upon document creation and calculate the new cost then added it to the collection "totals"

Answer 2

The following should do the trick:

firebase.firestore().collection("users").doc(uid).collection("services").get().then((snapshot) => {
         let serviceCostTotal = 0;
         snapshot.forEach((doc) => {
           serviceCostTotal = serviceCostTotal + parseInt(doc.data().serviceCost, 10);
         })
         // Do whatever you want with serviceCostTotal
    });

You need to loop over all the docs returned by the query and sum up the value of the field. Note that it is stored as a String, so you need to convert it to a number.

I make the assumption that the values of serviceCost are always an integer. If they are a float, it is recommended to use a library like Big.js when summing up.

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Note however that, with this approach, each time you want to calculate the sum you read the entire services subcollection for the user, which can be costly, depending on the number of Firestore documents in this subcollection. You could maintain an "external" sum in other Firestore document(s), as Methkal Khalawi proposes in his answer. Have a look at the Distributed Counters concept , for some inspiration for such a solution.

Answer 3

I guess I'm late but after some extensive research, there is a way in which we can fetch specific fields from firestore. We can use the select keyword, you're query would be somthing like (I'm using a collection for a generalized approach):

const result = await firebase.firestore().collection("users").select("serviceCost").get();

Where we can access the result.docs to further retrieved the returned filtered result. Thanks!