Swift中如何从FireStore按顺序查询单个指定字段数据？

Let's break down what's happening in your code and how to adjust it to get single usernames in the sorted order you want.

The Issue in Your Current Code #

Right now, you're looping through every document in your snapshot (limited to 5), and for each document, you're converting it to a UsersData instance and printing its name. That's why you see multiple names in your output—you're printing one per document in the query result.

Solution 1: Get Just the Top Leader's Username #

If you only need the single top user (sorted by BOOKS_Field descending), skip the loop and directly access the first document in the snapshot:

func getLeadersfromServer() {
    let leaderRef = db.collection("USERS_Collection").order(by: "BOOKS_Field", descending: true).limit(to: 5)
    leaderRef.addSnapshotListener { (snapShot, error) in
        if let error = error {
            print(error)
        } else {
            // Check if there are any documents available
            guard let topDocument = snapShot?.documents.first else {
                print("No users found in leaderboard")
                return
            }
            
            // Convert the top document to your UsersData model
            if let topUser = UsersData(dictionary: topDocument.data()) {
                print("Top Leader: \(topUser.NAME_FIELD)")
                // Store this username in a variable or use it directly here
            } else {
                print("Failed to parse top user data")
            }
        }
    }
}

Solution 2: Collect All Sorted Usernames to Access Individually #

If you want to keep all 5 sorted usernames and access them one by one (e.g., first, second, third place), collect them into an array first:

func getLeadersfromServer() {
    let leaderRef = db.collection("USERS_Collection").order(by: "BOOKS_Field", descending: true).limit(to: 5)
    leaderRef.addSnapshotListener { (snapShot, error) in
        if let error = error {
            print(error)
        } else {
            var sortedUsernames: [String] = []
            
            // Loop through documents and build the sorted list
            for document in snapShot?.documents ?? [] {
                if let user = UsersData(dictionary: document.data()) {
                    sortedUsernames.append(user.NAME_FIELD)
                }
            }
            
            // Now access single usernames in order
            if let firstPlace = sortedUsernames.first {
                print("1st Place: \(firstPlace)")
            }
            if sortedUsernames.count >= 2 {
                print("2nd Place: \(sortedUsernames[1])")
            }
            // Repeat for indices 2, 3, 4 as needed
        }
    }
}

Quick Note on Snapshot Listeners #

If you only need to fetch the data once (not listen for real-time updates), replace addSnapshotListener with getDocuments(completion:) instead.

内容的提问来源于stack exchange，提问作者Tom B.