Saving Data on Android

First Rule
: Collections can only contain documents. For example, you cannot add 
a 
String
 to the collection.
•
Second Rule
: Documents cannot contain other documents; however, they can 
point to subcollections. For example, your collections can contain many 
documents, and those documents can point to other collections. This is how things 
are formatted in a tree-like structure.
•
Third Rule
: The root of the Cloud Firestore database can only contain collections.
For example, in the 
WhatsUp
 app you created earlier, you could have a 
Posts
 
collection that contains a document for each post. Each document would point to a 
Comments
 collection that contains comments for that post, and the document that 
contains the comments would point to another collection, and so on.
When you worked with the Realtime Database, you learned that you should avoid 
these deeply nested hierarchy structures. In Cloud Firestore, however, these deeply 
nested structures are typical because the queries are shallow, meaning that querying 
data from a document will get you 
only that document
; you don’t have to query the 
entire collection or the subcollections within the document. This also means that 
queries are more efficient and flexible than in a Realtime Database, especially when 
it comes to filtering and sorting the data.
With 
WhatsUp
 app running with Cloud Firestore, you could have a collection of 
posts and any other collections you need to represent the data.