How to associate to different data objects in Firebase 3 with each other using Swift?

I know in SQL to associate 2 different objects with each other one would use a primary key in 1 table and a foreign key in another table. Since FirebaseDatabase uses JSON/NoSQL that's not possible. If I had 2 objects being UserPostEntity and PostEntity, once a user made a post/comment how would I associate a UserPostEntity with a PostEntity and how would the PostEntity be automatically updated with that post/comment the user made?

UserEntity Object:

import Foundation

    class UserEntity{

        var userID: String
        var postedComment: String
        var postDate: String
        var postID: PostEntity

        init(userID: String, postedComment: String, postDate: String, postID: PostEntity){
            self.userID = userID
            self.postedComment = postedComment
            self.postDate = postDate
            self.postID = postID
        }
    }

PostEntity Object:

import Foundation

class PostEntity{

    var userID: String
    var postID: String
    var postedComment: String
    var postDate: String

    init(userID: String, postID: String, postedComment: String, postDate: String){
        self.userID = userID
        self.postID = postID
        self.postedComment = postedComment
        self.postDate = postDate
    }
}

It would be better if you could structure your data in Firebase. Here is the links that provide nice intuition about structuring data in Firebase:

https://firebase.googleblog.com/2013/04/denormalizing-your-data-is-normal.html https://www.firebase.com/docs/web/guide/structuring-data.html

You can define your user table and posts as follow:

user{
    "u1" {
    
         userName : "abc",
         posts {
              p1 : true,
              p2 : true
          }
        },
        
        "u2" {
    
         userName : "def",
         posts {
              p3 : true,
              p4 : true
          }
        }
        
  }
  
  post{
        "p1"{
            userId : "u1",
            postComment : "hello ios",
            postDate : "1467570919"
            },
        "p2"{
            userId : "u1",
            postComment : "ios",
            postDate : "1467570920"
            },
        "p3"{
            userId : "u2",
            postComment : "hello ios",
            postDate : "1467570921"
            },
        "p4"{
            userId : "u2",
            postComment : "hello ios",
            postDate : "1467570922"
            }
    }

Also you can creates your entities as follow:

class UserEntity{

        var userID: String
        var userName : String
        var posts: Dictionary<String, Bool>?
        var ref : FIRDatabaseReference?

        init(userID: String, userName: String, posts: Dictionary<String, Bool>?){
            self.userID = userID
            self.userName = userName
            self.posts = posts
            self.ref = nil
        }
    }
    
    
    class PostEntity{

    var pId: String
    var uId: String
    var postedComment: String
    var postDate: NSTimeInterval
    var ref : FIRDatabaseReference?

    init(pId: String, uId: String, postedComment: String, postDate: NSTimeInterval){
        self.pId= pId
        self.uId = uId
        self.postedComment = postedComment
        self.postDate = postDate
        self.ref = nil
    }
}

Also you would want to structure your UserEntity and PostEntity entity as answered in this post.

You have to update the posts attribute of user table as p5 :true, when a new post p5 is added by user u1.