从Firebase检索键的特定值的列表 [英] Retrieve list of a particular value for a key from Firebase

问题描述

我具有以下firebase结构:

I have below firebase structure:

2014 {
 0 {
  CityState: "New York, NY",
  PinCode: "12345"
 },
 1 {
  CityState: "Los Angeles, CA",
  PinCode: "67890"
 }
}

我必须从firebase数据库中获取所有CityState字段的列表. 但是我总是获取完整的数据，然后不得不像下面这样过滤数据:

I have to get list of all the CityState field from the firebase database. But I always get full data and then have to filter the data like below:

ref?.child("2014").observeSingleEvent(of: .value, with: { snapshot in
            if let item = snapshot.value as? [Dictionary<String, Any>] {
                for items in item {
                    print(items["CityState"])
                }
            } else {
                print (snapshot.value)
            }
        })

由于要检索的海量数据，时间太长.是否有一个firebase查询，我可以从中获取"CityState"键的值列表?

It takes too long time because of huge data to be retrieved. Is there a firebase query where I can get list of values of "CityState" key?

我正在使用Swift 3.2和Xcode 9.2

I am using swift 3.2 and Xcode 9.2

推荐答案

Firebase始终从数据库返回完整的节点.没有API可以获取每个节点的子集.

Firebase always returns full nodes from the database. There is no API to get a subset of each node.

在使用Firebase数据库(和大多数其他NoSQL数据库)时，建议对应用程序用例的数据进行建模.因此，如果您需要在应用程序中显示城市州列表，则可能应该在数据库中存储城市州列表:

When using the Firebase Database (and most other NoSQL databases) it is recommended to model the data for the use-cases of your app. So if you need to show a list of city states in your app, you should probably store a list of city states in your database:

CityStates: {
 0: "New York, NY",
 1: "Los Angeles, CA"
}

如果您只想显示特定年份的城市，可能还需要按年份存储它们(就像您的原始JSON一样):

If you want to show the city states for only a specific year, it might be worth also storing them by year (as your original JSON does):

CityStates: {
  2014 {
   0: "New York, NY",
   1: "Los Angeles, CA"
 }
}

有了这些结构，从您的应用程序中读取数据变得微不足道，并且您读取的数据不超过您需要在应用程序中显示的数据.

With these structures, reading the data from your app becomes trivial and you're reading no more data than what you need to display in your app.

注意:以上结构以类似数组的结构存储城市状态.如果顺序不重要，请考虑将它们存储在类似集合的结构中:

Note: The above structure stores the city states in an array-like structure. If the order is not important, consider storing them in a set-like structure:

CityStates: {
  2014 {
   "Los Angeles, CA": true,
   "New York, NY": true
 }
}

此结构的优点是不会有任何重复项，因为根据定义，键在父节点中保证是唯一的.

The advantage of this structure is that there can't be any duplicates, since keys are by definition guaranteed to be unique within a parent node.

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