如何在Firebase中检索数据范围 [英] How to retrieve range of data in firebase

问题描述

我有以uuid为键的用户数据.我想向每个用户发送邮件，但是我不想一起向他们发送邮件，所以我想检索用户范围.

I have user's data which have uuid as their key. I want to send mail to each user, But I don't want to send them mail all together so I want to retrieve range of user.

例如，我现在有1000位用户，我想发送邮件到1-100位用户，然后是101-200位，201-300位，依此类推.我怎样才能做到这一点?我已经看过 startAt()和 endAt()函数，但是我的问题是我不知道开头和结尾的用户密钥.因此，我将无法通过这种方式检索距离.

For example, I have 1000 user's now I want to send mail to range 1-100 user then 101-200 , 201-300 and so on. How can I achieve this? I have seen startAt() and endAt() functions, but my question is I don't know user key at the beginning and at the end. So I won't be able to retrieve range from that way.

推荐答案

要获取前100个用户，您需要在以下位置进行查询:

To get the first 100 users you'd do a query on:

query = ref.orderByKey().limitToFirst(100)

然后，在处理用户时，请跟踪已处理的最后一个密钥:

Then as you process the users, keep track of the last key you've processed:

vast lastSeenKey;
query.once("value", function(snapshot) {
  snapshot.forEach(function(userSnapshot) {
    lastSeenKey = userSnapshot.key;
  });
});

然后，要加载接下来的100个用户，请使查询从您看到的最后一个键开始:

And then to load the next 100 users, you make the query start at the last key you've seen:

query = ref.orderByKey().startAt(lastSeenKey).limitToFirst(101)

您会注意到，我们在此处检索了一个额外的项目，因为此查询中的第一个用户将与上一个查询中的最后一个用户相同.

You'll note that we retrieve one extra item here, since the first user in this query will be the same as the last user in the previous query.

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