只从集合中检索特定的键 [英] Retrieve only specific keys from collection

问题描述

我想存储一组笔记，每个笔记都包含一个标题和一个正文。是否有可能检索没有身体的笔记标题列表？

例如：
$ b $ p $ code
注意：[
ewi4ef36wi：{
title：'Shopping List'，
body：'todo'
}
o9kng23inw：{
title：'要做的事'，
body：''
}
]

我怎样才能检索标题列表？

解决方案

不。 Firebase的Web / JavaScript API始终会返回您请求的节点下的完整树。

最常用的解决方法是，人们在树中只设置一个二级分支，只保留键。

 注意
 1：{body：hello，title：yessir} 
 2：{body：again，title：title2} 
 3：{body：there，title：another} 
 Notes_index 
 1：true 
 2：true 
 3：true 
  

通常被称为索引。你可以在 Notes_index 上加上（'child_added'），然后（如果需要）使用 once（'value'。

索引也常用于使节点可以通过备用密钥访问，例如标题索引：

  Title_index 
another：3 
title2：2 $ Firebase是b $ byessir：1 
  

这个最后的结构可能不再需要了，扩展他们的查询API以允许在任何字段上进行排序/过滤。但是对于你的用例，索引仍然是有用的。

I would like to store an array of notes, where each note contains a title and a body. Is it possible to retrieve the list of note titles without the body?

For example:

notes: [ ewi4ef36wi: { title: 'Shopping List', body: 'todo' } o9kng23inw: { title: 'Things To Do', body: '' } ]

How could I just retrieve the list of titles?

解决方案

Nope. Firebase's Web/JavaScript API always returns the full tree under the nodes that you request.

The most common workaround for this is that people set up a secondary branch in the tree where they just keep the keys.

Notes
    1: { "body": "hello", "title": "yessir" }
    2: { "body": "again", "title": "title2" }
    3: { "body": "there", "title": "another" }
Notes_index
    1: true
    2: true
    3: true

This is commonly referred to as an index. You'd on('child_added' on Notes_index and then (if needed) get the content of each note using once('value'.

Indexes are also often used to make nodes accessible by an alternative key. Such as a title index for the above:

Title_index
    "another": 3
    "title2": 2
    "yessir": 1

This last structure might soon not be needed anymore, since Firebase is extending their query API to allow ordering/filtering on any field. But for your use-case an index is still useful.

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