Thursday, February 18, 2016

Firebase - Queries

Firebase logo In the previous post I was discussing Firebase, used in Javascript, and that covered initialization, basic security, read all and insert. In this post I want to discuss about complex queries: filtering, ordering, limiting, indexing, etc. For that I will get inspiration (read: copy with impunity) from the Firebase documentation on the subject Retrieving Data, but make it quick and dirty... you know, like sex! Thank you, ma'am!

OK, the fluid interface for getting the data looks a lot like C# LInQ and I plan to work on a Linq2Firebase thing, but not yet. Since LInQ itself got its inspiration from SQL, I was planning to structure the post in a similar manner: how to do order by, top/limit, select conditions, indexing and so on, so we can really use Firebase like a database. An interesting concept to explore is joining, since this is an object database, but we still need it, because we want to filter by the results of the join before we return the result, like getting all the transaction of users that have the name 'Adam'. Aggregating is another thing that I feel Firebase needs to support. I don't want a billion records in order to compute the sum of a property.

However, the Firebase API is rather limited at the moment. You get .orderByChild, then stuff like .equalTo, .startAt and .endAt and then .limitToFirst and .limitToLast. No aggregation, no complex filters, no optimized indexing, no joining. As far as I can see, this is by design, so that the server is as dumb as possible, but think about that 1GB for the free plan. It is a lot.

So, let's try a complex query, see were it gets us.
  .once('value',function(snapshot) {
    var users=[];
    snapshot.forEach(function(childSnapshot) {
      var item=childSnapshot.val();
      if (/adam/i.test( {

function getInvoiceTotalForUsers(users,callback)
  var sum=0;
  var count=0;
  for (var i=0; i<users.length; i++) {
    var id=users[i];
      .once('value',function(snapshot) {
        snapshot.forEach(function(childSnapshot) {
          var item = childSnapshot.val();
          if (count==users.length) callback(sum);

First, I selected the users that have 'adam' in the name. I used .once instead of .on because I don't want to wait for new data to arrive, I want the data so far. I used .forEach to enumerate the data from the value event. With the array of userIds I call getInvoiceTotalForUsers, which gets all the invoices for each user, with a price bigger or equal to 10 and less or equal to 100, which finally calls a callback with the resulting sum of invoice prices.

For me this feels very cumbersome. I can think of several methods to simplify this, but the vanilla code would probably look like this.