根据月份的日期在 Firebase 中保留文档字段的总计

Question

I have Firebase collection of MeterReading which store electricity readings, here is what it look likes, suppose i have 3 records in collection 2 with date of 12 and 1 with date of 20 as following

meter_no : "A112"
reading : 6000
time : 12 May 2020 at 04:45:20 UTC+5


meter_no : "A112"
reading : 500
time : 12 May 2020 at 07:45:20 UTC+5

meter_no : "A134"
reading : 800
time : 20 May 2020 at 04:45:20 UTC+5

now i want to list down sum of reading per day as in our case (sum of readings of date 12 and sum of readings of date 20). How should i query this problem?

Answer 1

In Firestore, you cannot perform complex queries, since it's not built for that, so you could do this in 2 ways (Since you did not specify a language I will give examples in Javascript):

• Keep the firestore structure as is and operate programatically:

var snapshotAfter = await db.collection("readings")
    .where("time", ">=", "12 May 2020 at 00:00:00 UTC+5")
    .get()
    .catch(function(error) {
        console.log("Error getting documents: ", error);
});
var snapshotBefore = await db.collection("readings")
    .where("time", "=<", "13 May 2020 at 00:00:00 UTC+5")
    .get()
    .catch(function(error) {
        console.log("Error getting documents: ", error);
});
var list = [];
snapshotAfter.forEach(function(docAfter) {
     snapshotBefore.forEach(function(docBefore) {
        if(docAfter == docBefore){
            list.push(docAfter);
        }
     });   
});
var sum = list.length;

I find this code quite complex and not very efficient, the better solution would be the above.

• Change the Firestore Structure and do one simple query:

If you change you firebase structure to add a have a date field and an array of reading it will make it simpler to query, it could be something like this:

date: 12 May 2020 at 00:00:00 UTC+5
readingsArray: [
              {
                  meter_no : "A112"
                  reading : 6000
                  time : 12 May 2020 at 04:45:20 UTC+5
              },
              {
                  meter_no : "A112"
                  reading : 500
                  time : 12 May 2020 at 07:45:20 UTC+5
              }
          ]

date: 20 May 2020 at 00:00:00 UTC+5
readingsArray: [
              {
                  meter_no : "A134"
                  reading : 800
                  time : 20 May 2020 at 04:45:20 UTC+5
              }
          ]

And you can query your data by simply doing this:

var snapshot = await db.collection("readings")
    .where("date", "==", "12 May 2020 at 00:00:00 UTC+5")
    .get()
    .catch(function(error) {
        console.log("Error getting documents: ", error);
});
var sum = snapshot.docs[0].readingsArray.length;

So, in summary, it really depends on where you want to add complexity, the code or the db.

NOTE : All of this code is an untested example so you might have to refine it, but should be a good starting point.