在Firebase中检索常用值

Question

从Firebase中提取所有常用元素的最有效方法是什么？我正在尝试拉出postName相同的所有帖子并获取其各自的userId并将其附加到数组中。根据下面的数据库结构，输出应为[“userId1”，“userId2”，“userId4”，“userId5”]。

func fetchPosts() {

    let usersRef = Database.database().reference().child("posts")

    usersRef.observeSingleEvent(of: .value, with: { snapshot in

        for child in snapshot.children.allObjects as! [DataSnapshot] {

            let value = child.value as? NSDictionary

            let postName = value?["postName"] as? String ?? ""

            //check for postName that have the same value
            //for each of those postName, fetch their relevant userId
        }
    })
}

数据库结构

_Posts
__postId1
____postName: "Common"
____userId: "userId1"
__postId2
____postName: "Common"
____userId: "userId2"
__postId3
____postName: "Not common"
____userId: "userId3"
__postId4
____postName: "Also common"
____userId: "userId4"
__postId5
____postName: "Also common"
____userId: "userId5"