Sort document by integer Firestore Kotlin - firebase

Question

I want to sort documents by img in descending order.
I've tried to add indexes in firebase console. Here is a screenshot from indexes tab:
Here is my database structure:
Here is my code:
db.collection("images").whereEqualTo("user", uid).orderBy("img", Query.Direction.DESCENDING).get()
.addOnSuccessListener {
if (it.isEmpty) {
Log.i("Image", "Sorry, no image")
} else {
for (task in it) {
Log.i("Image", task.get("img").toString())
}
}
}
And here is log:
I/Image: 2
I/Image: 1
I/Image: 3
I/Image: 4
I/Image: 6
I/Image: 7
What am I doing wrong?

Answer 1

Notice that as stated on the documentation there could be two explanations behind the behavior you are experiencing:
If you have a filter with a range comparison (<, <=, >, >=), your first ordering must be on the same field
You cannot order your query by any field included in an equality (=) or in clause.
Based on the following snippet provided on the documentation:
citiesRef.whereGreaterThan("population", 100000).orderBy("population").limit(2)
You could try the following (but I'm not sure if it'll work because of 2.):
db.collection("images").whereEqualTo("user", uid).orderBy("user").orderBy("img", Query.Direction.DESCENDING).get()
.addOnSuccessListener {
if (it.isEmpty) {
Log.i("Image", "Sorry, no image")
} else {
for (task in it) {
Log.i("Image", task.get("img").toString())
}
}
}