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并将带有某些查询的FirebaseRecyclerOptions传递给构造函数.每个项目都有许多字段，包括时间戳记a和另一个整数字段b.因此，我希望将a较大但b较少的项目放在列表的顶部.使用sqlite易于实现，而使用nosql则不那么容易.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?

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?

无法加载整个列表并使用Java进行排序.

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

推荐答案

不幸的是，Firebase Realtime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，您猜测需要创建一个额外的字段来保留这两个字段是正确的.因此，要实现此目的，您需要创建一个新字段，该字段在数据库中应如下所示:

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

因此，如您所见，a_b属性将要过滤的值组合在一起.

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

与Firebase Realtime数据库不同， Cloud Firestore 允许

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");

订购字符串时，这是正常的订购方法.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，例如:"1516428687_001"，"1516428687_002"，"1516428687_012".现在您的订单就可以了.有关更多说明，请参见此发布.

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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