Firebase Android：push().getKey() 但在开头插入元素，而不是在结尾

Question

String key = databaseReference.child("Photo").push().getKey();
// ...
databaseReference.child("Photo").child(key).setValue(post);

据我所知，Firebase 在这里创建了一个有统计保证的唯一 ID，并且它还有一个订单。我查了一下，确实有订单。像这样，它总是在数据结构的末尾插入新元素。比如这样：

-MqWU0En0OezPglE6SfL: true
-MqWUFoFqhIaT6fSk-DT: true
-MrmJaNZ8lbcOFPodTBO: true

变成：

-MqWU0En0OezPglE6SfL: true
-MqWUFoFqhIaT6fSk-DT: true
-MrmJaNZ8lbcOFPodTBO: true
-Mrq162Eba8KHgc_B0lh: true (New Element was inserted here at the end!)

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我需要反其道而行之。我需要在开头插入它。有没有办法做到这一点？也许像push().getKeyAtBeginning() 这样的东西？我找不到任何东西。

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如果没有，你有什么建议？

在孩子中，photo 是成千上万张照片。我知道您可以在每张具有-1 * currentMillis 值的照片中插入一个子元素negativetimestamp，然后只需使用orderByChild(negativetimestamp)。但我认为这不是一个好主意，因为届时 Firebase 必须订购成千上万甚至上万张照片。

Answer 1

不幸的是，你无法使用这样的东西：

push().getKeyAtBeginning()

您不能将节点添加到特定索引。除此之外，您也无法更改Firebase Console 中节点的顺序。所有节点默认按key排序。

如果您需要按特定标准对元素进行排序，那么您的想法就是前进的道路。我也回答了一个类似的问题：

• Sort data to RecyclerView based on latest date from Firebase

如果有数千甚至一万张照片，那么您应该考虑将它们分成更小的块。这种方法称为pagination。

Answer 2

此答案中的旧代码包含错误！因此，我将其彻底编码为新代码。

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Firebase 使用时间戳来确保生成的密钥的时间顺序。 我编写了一个按字典顺序反转键的方法（以双射方式），因此两个键之间的所有顺序关系仍然存在，但相反。所以如果key1 > key2，那么在key1

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新代码：

import java.util.HashMap;
import java.util.Map;

public class InvertiereKey {

    /**
     * all allowed chars for a firebase key in a lexicographical order (except SPACE)
     */
    private static Map<Integer, String> hashtable;

    private static void initHash() {
        if (hashtable == null) {
            hashtable = new HashMap<>();

            hashtable.put(0, "!");
            hashtable.put(1, "\"");
            hashtable.put(2, "%");
            hashtable.put(3, "'");
            hashtable.put(4, "(");
            hashtable.put(5, ")");
            hashtable.put(6, "*");
            hashtable.put(7, "+");
            hashtable.put(8, ",");
            hashtable.put(9, "-");
            //hashtable.put(10, "/"); // This does NOT work because child("a/b") means child("a").child("b")
            hashtable.put(10, "0");
            hashtable.put(11, "1");
            hashtable.put(12, "2");
            hashtable.put(13, "3");
            hashtable.put(14, "4");
            hashtable.put(15, "5");
            hashtable.put(16, "6");
            hashtable.put(17, "7");
            hashtable.put(18, "8");
            hashtable.put(19, "9");
            hashtable.put(20, ":");
            hashtable.put(21, ";");
            hashtable.put(22, "<");
            hashtable.put(23, "=");
            hashtable.put(24, ">");
            hashtable.put(25, "?");
            hashtable.put(26, "@");
            hashtable.put(27, "A");
            hashtable.put(28, "B");
            hashtable.put(29, "C");
            hashtable.put(30, "D");
            hashtable.put(31, "E");
            hashtable.put(32, "F");
            hashtable.put(33, "G");
            hashtable.put(34, "H");
            hashtable.put(35, "I");
            hashtable.put(36, "J");
            hashtable.put(37, "K");
            hashtable.put(38, "L");
            hashtable.put(39, "M");
            hashtable.put(40, "N");
            hashtable.put(41, "O");
            hashtable.put(42, "P");
            hashtable.put(43, "Q");
            hashtable.put(44, "R");
            hashtable.put(45, "S");
            hashtable.put(46, "T");
            hashtable.put(47, "U");
            hashtable.put(48, "V");
            hashtable.put(49, "W");
            hashtable.put(50, "X");
            hashtable.put(51, "Y");
            hashtable.put(52, "Z");
            hashtable.put(53, "\\");
            hashtable.put(54, "^");
            hashtable.put(55, "_");
            hashtable.put(56, "`");
            hashtable.put(57, "a");
            hashtable.put(58, "b");
            hashtable.put(59, "c");
            hashtable.put(60, "d");
            hashtable.put(61, "e");
            hashtable.put(62, "f");
            hashtable.put(63, "g");
            hashtable.put(64, "h");
            hashtable.put(65, "i");
            hashtable.put(66, "j");
            hashtable.put(67, "k");
            hashtable.put(68, "l");
            hashtable.put(69, "m");
            hashtable.put(70, "n");
            hashtable.put(71, "o");
            hashtable.put(72, "p");
            hashtable.put(73, "q");
            hashtable.put(74, "r");
            hashtable.put(75, "s");
            hashtable.put(76, "t");
            hashtable.put(77, "u");
            hashtable.put(78, "v");
            hashtable.put(79, "w");
            hashtable.put(80, "x");
            hashtable.put(81, "y");
            hashtable.put(82, "z");
            hashtable.put(83, "{");
            hashtable.put(84, "|");
            hashtable.put(85, "}");
            hashtable.put(86, "~");
        }
    }

    
    /**
     * um andere einfuege reihenfolge von push().getKey() bei firebase zu haben
     * Also am anfang wird eingefuegt dann, nicht ans ende
     *
     * @param key
     * @return
     */
    public static String invertiereKey(String key) {
        initHash();

        String invertiert = "";

        // get char array
        char[] charArray = key.toCharArray();

        // iterate over it and invert every char lexicographically
        int i = 0;
        while (i < charArray.length) {
            char charTemp = charArray[i];

            // get numerical value
            int numerical = getNumericalFromChar(charTemp);

            // invert the numerical representation
            // there are 88 chars
            // 0 --> first char, 87 --> last char
            int numericalNeu = 87 - numerical;

            // get char again
            String newChar = hashtable.get(numericalNeu);

            // add to inverted string
            invertiert += newChar;

            i++;
        }

        return invertiert;
    }

    private static int getNumericalFromChar(char c) {
        String s = Character.toString(c);

        for (Map.Entry<Integer, String> entry : hashtable.entrySet()) {
            if (s.equals(entry.getValue())) {
                return entry.getKey();
            }
        }

        return 0; // never called with proper input (ie. a firebase key generated from pushKey())
    }
}

用法：

String key = reference.pushKey();
String invertedKey = InvertiereKey.invertiereKey(key);
reference.child(invertedKey).setValue(...);