如何使用查询或特定路径从 Firebase 实时数据库中高效查询

Question

We have infrequently property modification("price") on Firebase Realtime Database which is structure like this:

../currencies/<currency>/value/
      "price":343

every player that log in will listen to only one and specific currency. our client will pick the right currency path based on the player preferences.

so if player is set to currency USD the firebase client will listen to this path

../currencies/USD/value/
      "price":343

Those currencies prices are changed infrequently
due to this structure our server side need to modify and denormalized the data to all currencies when it changes (we can have ten's of currencies) because of that we add to the leaf even more properties which are identical at all currencies

which I find redudent like:

../currencies/USD/value/
          "price":343
          "currency-source":"fx" . //this property will be copied to all 

currencies vals because the client listen to only one path and it needs this data aswell

instead if manitain this on path's perhaps we can use some query where each client will be able to pick it's currency based on property name?

something like that:

../currencies/value/
          "USD_price":343
          "EUR_price":343
      ...

thoughts about design? and if sounds better how can be achieved with Firebase Realtime Database query?

Answer 1

I don't know which language (which Client SDK) you use in your app, but here is a solution with the JavaScript SDK.

Let's imagine you have a Relatime Database structure as follows

  "parentnode" : {
    "currencies" : {
      "EUR" : {
        "value" : {
          "price" : 201
        }
      },
      "USD" : {
        "value" : {
          "price" : 343
        }
      },
      "value" : {
        "EUR_price" : 201,
        "USD_price" : 343,
        "currency-source" : "fx"
      }
    }
  }

Under a parentnode you have a currencies node which corresponds to the examples in your question .

In case you want to listen to /currencies/<currency>/value , you would do as follows:

  var db = firebase.database();

  var currency = 'EUR';

  var ref = db.ref().child('parentnode/currencies/' + currency);
  ref.on('value', function(snapshot) {
    var data = snapshot.val();
    console.log(data);
  });

In case you want to listen to /currencies/value/<currency>_price and get the price and the currency-source values, you would do as follows:

  var db = firebase.database();

  var currency = 'EUR';

  var ref = db.ref().child('parentnode/currencies/value');
  ref.on('value', function(snapshot) {
    var data = snapshot.val();
    var price = data[currency + '_price'];
    var source = data['currency-source'];
    console.log(price);
    console.log(source);
  });

---

As mentioned in your comment, the second approach implies that "we will download all the data leaf under /currencies/value/ ".

I can think of two other possible approaches. Choosing one over the other really depends on your functional requirements, ie what you do with these values in your front-end.

1/ Set two listeners

The idea is to set one listener for 'parentnode/currencies/value/' + currency + '_price' and one for 'parentnode/currencies/value/currency-source' , as follows:

  var currency = 'EUR';

  var ref2 = db
    .ref()
    .child('parentnode/currencies/value/' + currency + '_price');
  ref2.on('value', function(snapshot) {
    var data = snapshot.val();
    console.log(data);
  });

  var ref3 = db.ref().child('parentnode/currencies/value/currency-source');
  ref3.on('value', function(snapshot) {
    var data = snapshot.val();
    console.log(data);
  });

2/ Query the currency-source value within the listener

With this second approach, in the listener to 'parentnode/currencies/value/' + currency + '_price' , we query the database with the once() method to get the value of currency-source :

  var ref4 = db
    .ref()
    .child('parentnode/currencies/value/' + currency + '_price');
  ref4.on('value', function(snapshot) {
    var data = snapshot.val();
    console.log(data);
    db.ref()
      .child('parentnode/currencies/value/currency-source')
      .once('value')
      .then(function(dataSnapshot) {
        console.log(dataSnapshot.val());
      });
  });

---

Note that if you do not need to set a listener, ie you want to fetch the data once (eg by triggering the fetch from a button, or on page loading, etc..) you should only use the once() method and then you can chain the two calls as follows:

  var currency = 'EUR';
  var ref5 = db.ref().child('parentnode/currencies/value/' + currency + '_price');
  var ref6 = db.ref().child('parentnode/currencies/value/currency-source');

  ref5
    .once('value')
    .then(function(dataSnapshot) {
      console.log(dataSnapshot.val());
      return ref6.once('value');
    })
    .then(function(dataSnapshot) {
      console.log(dataSnapshot.val());
    });