Firebase - 从数据库节点获取随机子节点 [英] Firebase - Get Random Child From Database Node
问题描述
我正在编写 Android 应用程序中调用的 Firebase云功能。该函数应该从数据库中的用户节点获取随机用户（任何随机用户，但不是发送请求的用户）并将其返回给客户端。问题是数据没有结构化为数组（它不应该也不可能）。
I'm writing a Firebase Cloud Function that is called from an Android app. The function should get a random user from the users node in the Database (Any random user but not the one who sent the request) and return it to the client. The problem is that the data is not structured as an array (And it should not be and can't be).
所以我不能选择一个随机数并读取该节点，我也不想读取所有用户节点并迭代它，因为它包含大量用户，这将耗费时间和资源。 问题：
So I can't pick a random number and read that node, I also don't want to read all the users node and iterate it because it contains a lot of users and that would be time and resources-consuming. The Question:
如何让随机用户与发送请求的用户不同，并且更改每次调用函数时（完全随机），而不是从数据库中查询大量数据？
How can I get a random user different from the one who sent the request, and changes with every time the function is called (completely random), while not querying a lot of data from the database?
推荐答案
主要问题：
Firebase无法跟踪随机化所需的子计数。但是我们可以使用 DataSnapshot.numChildren（）。
为避免这种情况，您可以添加将元素计入 / users 节点，您必须使用云函数手动维护该节点（例如，添加用户时，增量 count ，并在删除时减少。）
To avoid this, you can add a count element to the /users node, which you would have to maintain manually, using cloud functions (e.g. when adding a user, increment count, and decrement on deletion).
另一方面，由于云功能在服务器上， firebase将为您管理缓存，实际使用 DataSnapshot.numChildren（）并不是什么大事，因为一旦为该函数执行了该函数，数据将被缓存第一次，只有在有变化时才会更新。
On the other hand, since the cloud functions are on the server, and firebase will manage the cache for you, it is not a big deal to actually use DataSnapshot.numChildren(), since the data will be cached once the function has been executed for the first time, and will be updated only when there are changes.
db.ref('/users').once('value').then(snapshot =>
Math.floor((Math.random() * snapshot.numChildren()))
);
或者使用计数 -method：
Alternatively using the count-method:
db.ref('/users/count').once('value').then(snapshot =>
Math.floor((Math.random() * snapshot.val()))
);
获取随机节点：
< s>
Getting the random Node:
db.ref('/users').orderByKey().startAt(random).limitToFirst(1);
// https://stackoverflow.com/a/38423694/4161937
// attach permanent listener to force firebase caching (not sure if it works for this case)
db.ref('/users').on('value', () => {});
// will read #random amount of items
db.ref('/users').orderByKey().limitToFirst(random).once('value').then(users => Object.keys(users)[random]);
方法2：
Method 2:
// https://stackoverflow.com/a/38423694/4161937
// attach permanent listener to force firebase caching
db.ref('/users').on('value', () => {});
db.ref('/users').once('value').then(users => Object.keys()[random])
您可以在官方文档中阅读有关查询订单和过滤的更多信息：
You can read more about query order- and filtering in the official docs:
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