`Query.startAfter(Double value)` returns one more document than expected.

[russellwheatley]

[READ] Step 1: Are you in the right place?

Issues filed here should be about bugs in the code in this repository.
If you have a general question, need help debugging, or fall into some
other category use one of these other channels:

• For general technical questions, post a question on StackOverflow
  with the firebase tag.
• For general Firebase discussion, use the firebase-talk
  google group.
• For help troubleshooting your application that does not fall under one
  of the above categories, reach out to the personalized
  Firebase support channel.

[REQUIRED] Step 2: Describe your environment

• Android Studio version: Android Studio Artic Fox 2020.3.1 Patch 4
• Firebase Component: Database
• Component version: 29.0.4

[REQUIRED] Step 3: Describe the problem

Querying the database using the startAfter(double value) API is not returning the correct amount of documents (specifically returning one more than it should).

Steps to reproduce:

1. Run flutter create testProject.
2. Update "lib/main.dart" with the code under the "Relevant Code" header & input your firebase options as described in the code comment.
3. Run flutter app on an android device, the logs will output:

{search: 3}
{search: 4}
{search: 5}

4. Run flutter app on an iOS device will produce the correct output:

{search: 4}
{search: 5}

The relevant android code where we call startAfter is here

Relevant Code:

import 'dart:async';

import 'package:firebase_core/firebase_core.dart';
import 'package:firebase_database/firebase_database.dart';
import 'package:flutter/material.dart';

import 'firebase_options.dart';

Future<void> main() async {
  WidgetsFlutterBinding.ensureInitialized();
  await Firebase.initializeApp(
 
    options: FirebaseOptions(
		// <INSERT YOUR OWN FIREBASE OPTIONS TAKEN FROM "google-services.json" FILE>		
	),
  );

  runApp(
     MaterialApp(
      title: 'Flutter Database Example',
      home: MyHomePage(),
    ),
  );
}

class MyHomePage extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: const Text('Flutter Database Example'),
      ),
      body: Column(
        children: [
          ElevatedButton(
            onPressed: () async {
				//CREATES 5 DOCUMENTS IN DATABASE
              DatabaseReference _testsRef = FirebaseDatabase.instance.ref('tests');
              await _testsRef.set({
                'yo':{'search':2,},
                'so':{'search':3,},
                'to':{'search':4,},
                'fu':{'search':1,},
                'bo':{'search':5,},
              });
			// QUERY THAT SHOULD RETURN 2 DOCUMENTS IF STARTING AFTER 3 (THE THIRD DOCUMENT)
             DataSnapshot response = await _testsRef.orderByChild('search').startAfter(3).get();

             response.children.forEach((element) {
				// PRINTS OUT THE DOCUMENTS RETURNED
               print(element.value.toString());
             });
            },
            child: const Text('Press me'),
          ),
        ],
      ),
    );
  }
}

Hopefully, this makes sense as a flutter implementation! It is very straight forward. In a nutshell;

1. Create 5 documents.
2. Use orderByChild(String path) & startAfter(Double value) to query documents, and note that the query returns one more document than expected.

Let me know if you need any further information.

[argzdev]

Hi @russellwheatley, thanks for reporting. Unfortunately, I'm unable to reproduce the issue when I created a minimal repro of your issue with native Android. I was able to get the same results of what should happen on an iOS device.

Logs:

D/MainActivity: Update complete
D/MainActivity: test: DataSnapshot { key = tests, value = {to={search=4}, bo={search=5}} }
D/MainActivity: test: DataSnapshot { key = bo, value = {search=5} }
D/MainActivity: test: DataSnapshot { key = to, value = {search=4} }

Relevant code:

fun test(view: View){
        val database = Firebase.database
        val testRef = database.getReference("tests")

        testRef.setValue(mapOf(
            "yo" to mapOf("search" to 2),
            "so" to mapOf("search" to 3),
            "to" to mapOf("search" to 4),
            "fu" to mapOf("search" to 1),
            "bo" to mapOf("search" to 5),
        ))

        Log.d(TAG, "Update complete")

        testRef.orderByChild("search").startAfter((3).toDouble()).get().addOnSuccessListener { snapshot ->
            Log.d(TAG, "test: $snapshot")
            snapshot.children.forEach { child ->
                Log.d(TAG, "test: $child")
            }
        }
        .addOnFailureListener{
            Log.d(TAG, "exception: $it")
        }
    }

I did notice in the startAfter a double parameter is required, I'm not sure if this can somehow impact the query, but just something to take note of.

This issue may be more of within the Flutter sdk itself. Could you provide more details about the issue so we can investigate this further?

Also, may I ask what's the equivalent Android version of Firebase RTDB of Flutter being used? Thanks in advance!

[russellwheatley]

Hey @argzdev, I've just recreated this issue using java:

		DatabaseReference testRef = FirebaseDatabase.getInstance().getReference("tests");
        String TAG = "TTTTTTTTTT";
        Map<String, Number> ob1 = new HashMap();
        Map<String, Number> ob2 = new HashMap();
        Map<String, Number> ob3 = new HashMap();
        Map<String, Number> ob4 = new HashMap();
        Map<String, Number> ob5 = new HashMap();

        ob1.put("search", 1);
        ob2.put("search", 3);
        ob3.put("search", 2);
        ob4.put("search", 5);
        ob5.put("search", 4);

        Map<String, Map<String, Number>> oby = new HashMap();

        oby.put("yo", ob1);
        oby.put("to", ob2);
        oby.put("lo", ob3);
        oby.put("fo", ob4);
        oby.put("eo", ob5);

        Tasks.await(testRef.setValue(oby));

        
        Log.d(TAG, "Update complete");

        Number dub = 3;
        testRef.orderByChild("search").startAfter(dub.doubleValue()).get().addOnCompleteListener(new OnCompleteListener<DataSnapshot>() {
          @Override
          public void onComplete(@NonNull Task<DataSnapshot> task) {
            if (!task.isSuccessful()) {
              Log.e(TAG, "Error getting data", task.getException());
            } else {
              Map<String, Map<String, Number>> result = (Map<String, Map<String, Number>>) task.getResult().getValue();
              Log.d(TAG, "onComplete: " + result.toString());
            }
          }
        });

and I got the following log:

D/TTTTTTTTTT(25687): onComplete: {fo={search=5}, eo={search=4}, to={search=3}}

We are using the latest Firebase BOM (29.3.0). Not sure what is different 😅

[argzdev]

Hi @russellwheatley, thanks for the repro. I was able to replicate the issue.

It turns out there was a race condition which when I was uploading data and then the search was happening is somewhat causing issues on the results.

I'm now able to reproduce the issue on both Kotlin and Java.
I'll notify an engineer to take a look at this. Thanks!

[TarekkMA]

Any updates on this issue?