如何按两个字段对 Firebase 记录进行排序 (Android) [英] How to sort Firebase records by two fields (Android)

问题描述

我正在使用 FirebaseRecyclerAdapter 并将带有一些查询的 FirebaseRecyclerOptions 传递给构造函数.每个项目都有多个字段，包括时间戳 a 和另一个整数字段 b.所以我希望具有较旧 a 但较少 b 的项目位于列表顶部.用sqlite很容易实现，但用nosql就没有那么容易了.看来我需要添加一些额外的字段来保留两个字段:String.valueOf(a) + String.valueOf(b) 但是如何实现 asc/desc 属性呢?

I'm using FirebaseRecyclerAdapter and pass FirebaseRecyclerOptions with some query into constructor. Each item has number of fields including timestamp a and another integer field b. So I want items with older a but less b to be on top of list. It's easy to implement with sqlite, but not so easy with nosql. It seems I need to add some extra field which keeps both fields: String.valueOf(a) + String.valueOf(b) but how to achieve asc / desc properties?

加载整个列表并在 Java 中排序不是一种选择.

Loading the whole list and sorting in Java is not an option.

推荐答案

遗憾的是，Firebase 实时数据库不支持对多个属性的查询，仅支持对单个子属性的查询.因此，您正确地猜测您需要创建一个额外的字段来保留这两个字段.因此，要实现这一点，您需要在数据库中创建一个新字段，该字段应如下所示:

Unfortunately, Firebase Realtime database does not support queries on multiple properties, supports only queries on a single child property. So, you're correct in guessing that you'll need to create an extra field to keep both fields. So to achieve this, you need to create a new field which in your database should look like this:

Firebase-root
   |
   --- itemId
          |
          --- a: valueOfA
          |
          --- b: valueOfB
          |
          --- a_b: valueOfA_valueOfB

如您所见，a_b 属性组合了您要过滤的值.

So as you see, the a_b property combines the values that you want to filter on.

与 Firebase 实时数据库不同，Cloud Firestore 允许 复合查询.你应该看看这个.因此，Cloud Firestore 中允许使用以下查询，而无需创建组合属性.

Unlike Firebase Realtime database, Cloud Firestore allows compound queries. You should take a look at this. So a query as the one below is allowed in Cloud Firestore without creating a combined property.

itemIdRef.whereEqualTo("a", "valueOfA").whereEqualTo("b", "valueOfB");

当您对字符串进行排序时，这是正常的排序方法.一个快速的解决方法是在每个第二个元素之前添加两个零，如下所示:"1516428687_001", "1516428687_002", "1516428687_012".现在你的订单会很好.有关更多解释，请参阅我的回答 发布.

When you order strings this is the normal ordering method. A quick fix would be to add two zeros before each second element like this: "1516428687_001", "1516428687_002", "1516428687_012". Now your order will be fine. For more explanations, please see my answer from this post.

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