Multiple orderBy in firestore - firebase

Question

I have a question about how multiple orderBy works.
Supposing these documents:
collection/
doc1/
date: yesterday at 11:00pm
number: 1
doc2/
date: today at 01:00am
number: 6
doc3/
date: today at 13:00pm
number: 0
If I order by two fields like this:
.orderBy("date", "desc")
.orderBy("number", "desc")
.get()
How are those documents sorted? And, what about doing the opposite?
.orderBy("number", "desc")
.orderBy("date", "desc")
.get()
Will this result in the same order?
I'm a bit confused since I don't know if it will always end up ordering by the last orderBy.

Answer 1

In the documentation for orderBy() in Firebase it says this:
You can also order by multiple fields. For example, if you wanted to order by state, and within each state order by population in descending order:
Query query = cities.orderBy("state").orderBy("population", Direction.DESCENDING);
So, it is basically that. With logic from SQL where you have ORDER BY to order your table. Let's say you have a database of customers who are from all over the world. Then you can use ORDER BY Country and you will order them by their Country in any order you want. But if you add the second argument, let's say Customer Name, then it will first order by the Country and then within that ordered list it will order by Customer Name. Example:
1. Adam | USA |
2. Jake | Germany |
3. Anna | USA |
4. Semir | Croatia |
5. Hans | Germany |
When you call orderBy("country") you will get this:
1. Semir | Croatia |
2. Jake | Germany |
3. Hans | Germany |
4. Adam | USA |
5. Anna | USA |
Then when you call orderBy("customer name") you get this:
1. Semir | Croatia |
2. Hans | Germany |
3. Jake | Germany |
4. Adam | USA |
5. Anna | USA |
You can see that Hans and Jake switched places, because H is before J but they are still ordered by the Country name. In your case when you use this:
.orderBy("date", "desc")
.orderBy("number", "desc")
.get()
It will first order by the date and then by the numbers. But since you don't have the same date values, you won't notice any difference. This also goes for the second one. But let's say that one of your fields had the same date, so your data looks like this:
collection/
doc1/
date: yesterday at 11:00pm
number: 1
doc2/
date: today at 01:00am
number: 6
doc3/
date: today at 01:00am
number: 0
Now, doc2 and doc3 are both dated to today at 01:00am. Now when you order by the date they will be one below the other, probably doc2 will be shown first. But when you use orderBy("number") then it will check for numbers inside the same dates. So, if its just orderBy("number") without "desc" you would get this:
orderBy("date");
// output: 1. doc1, 2. doc2, 3. doc3
orderBy("number");
// output: 1. doc1, 2. doc3, 3. doc2
Because number 0 is before 6. Just reverse it for desc.