Firebase and reading nested data using Swift

var postsCommentsDict : NSMutableDictionary = NSMutableDictionary()
var userNameArray : [String] = [String]() 
var userCommentArray : [String] = [String]()
FIRDatabase.database.reference().child("Posts").observeEventType(.Value, withBlock: {(Snapshot) in

if Snapshot.exists(){
    let imageD = Snapshot.value
    let imageD_ID = imageD.key
   //Retrieving the email and image name
     let imageName = imageD["userImage"] as! String
     let userEmail = imageD["userEmail"] as! String
    //Here you are accessing each image ID
    //First Question Alternative Completed

         //To answer the second Question :-

     FIRDatabase.database.reference().child("Posts").child(imageD_ID).child("comments").observeEventType(.Value, withBlock: {(Snapshot) in

  if let commentsDictionary = Snapshot.value {

    for each in commentsDictionary{

      postsCommentsDict.setObject(each["userName"] as! String , forKey : each["userComment"] as! String)
               //Saving the userName : UserComment in a dictionary
      userNameArray.append(each["userName"] as! String)
      userCommentArray.append(each["userComment"] as! String)
               //Saving the details in arrays
               //Prefer dictionary over Arrays

    }

} else {

   //no comments to append the arrays

        }

      })

   }
})

Once you are Done Saving the Comments dictionary : How to read it : -

for each in postsCommentsDict as! NSDictionary{
  let userNm = each.key
  let userComment = each.value
  //username and userComment's retrieved 

}

Please ignore the Typos, if any!...hope this helps..