How to change Firestore query depending on user selection?

Question

I have an app that contains categories list in recyclerview. I also have recyclerview that shows posts based on selected categories. These 2 recyclerviews are in one fragment. What I want to achieve is that when choosing particular category lets suppose "Business" I want to show posts from that specific category. So far, I tried something like this but to no avail:

Query query = postRef.whereEqualTo("postCategory", selectedCategory).orderBy("createdAt", Query.Direction.DESCENDING);