Saving Data on Android

Listening for comments
You can add comments to the database now, but you still can't read them. Since 
comments are stored in a separate collection from posts, to read them you'll need to 
write a query that returns comments for the specific post. Every comment document 
has a 
post_id
 property that indicates to which post the comment belongs to.
Open 
CloudFirestoreManager.kt
. Add a 
commentsRegistration
 field:
private
 
lateinit
 
var
 commentsRegistration: ListenerRegistration
You'll use this field to assign the comments listener to it and to remove the listener 
when needed, just like before.
Next, navigate to 
listenForPostCommentsValueChanges()
. Replace the 
TODO
 
inside the function with the following:
// 1
 
commentsRegistration = database.collection(COMMENTS_COLLECTION) 
    
// 2
 
    .whereEqualTo(POST_ID, postId) 
// 3
 
    
// 4
 
    .addSnapshotListener(EventListener { value, 
error -> 
      
if
 (error != 
null
 || value == 
null
) { 
        
return
@EventListener
 
      } 
 
      
if
 (value.isEmpty) { 
        postsValues.postValue(emptyList()) 
      } 
else
 { 
        
val
 comments = ArrayList() 
        
for
 (doc 
in
 value) { 
          
val
 comment = doc.toObject(Comment::
class
.
java
) 
          comments.add(comment) 
        } 
        commentsValues.postValue(comments) 
      } 
    })
Saving Data on Android
Chapter 18: Reading Data from Cloud Firestore
raywenderlich.com
267

---

1. As before, you assing a listener to the 
comments
 reference.
2. You use 
whereEqualTo
 method to create the query that filters the documents in 
the collection that contain the specified field and value in that field.
3. Pass in the 
POST_ID
 that represents the 
post_id
 property and the 
postId
 that 
represents the value for comparison. This query will only return the documents 
that belong to the specified post. 
whereEqualTo
 method returns a 
Query
 that you 
can read or listen to.
4. Once again, attach an 
EventListener
 and parse the 
Comment
s, if there are any, 
updating the UI when you're done.
Next, open