本頁面由 Cloud Translation API 翻譯而成。

寫入時間匯總

您可以透過 Cloud Firestore 查詢尋找文件大型集合中。為了深入瞭解可以匯總集合的資料。

您可以在讀取時或寫入時匯總資料：

讀取時間匯總功能會在要求時計算結果。 Cloud Firestore 支援 count()、sum() 和 average() 匯總查詢。讀取時間匯總查詢更簡單而不是寫入時間匯總如要進一步瞭解匯總查詢，請參閱使用匯總查詢匯總資料。
寫入時間匯總會在應用程式每次執行時計算結果相關的寫入作業寫入時間匯總較有實作上的工作但針對其中一個。
- 您想監聽匯總結果來即時更新。不支援 count()、sum() 和 average() 匯總查詢即時更新。
- 您想將匯總結果儲存在用戶端快取中。不支援 count()、sum() 和 average() 匯總查詢快取功能。
- 正在匯總各個資料集的數萬份文件的資料並考量成本少量文件的讀取時間匯總費用較低如果是匯總作業中的大量文件寫入時間匯總的費用可能較低。

您可以使用用戶端或用戶端執行寫入時間匯總交易或 Cloud Functions以下各節��明如何執行寫入時間匯總

解決方案：透過用戶端交易進行寫入時間匯總

假設有一款當地推薦應用程式，可協助使用者找到美味的餐廳。以下查詢會擷取特定餐廳的所有評分：

網路

db.collection("restaurants")
  .doc("arinell-pizza")
  .collection("ratings")
  .get();test.solution-aggregation.js

Swift

注意：這項產品不適用於 watchOS 和 App Clip 目標。

do {
  let snapshot = try await db.collection("restaurants")
    .document("arinell-pizza")
    .collection("ratings")
    .getDocuments()
  print(snapshot)
} catch {
  print(error)
}SolutionAggregationViewController.swift

Objective-C

注意：這項產品不適用於 watchOS 和 App Clip 目標。

FIRQuery *query = [[[self.db collectionWithPath:@"restaurants"]
    documentWithPath:@"arinell-pizza"] collectionWithPath:@"ratings"];
[query getDocumentsWithCompletion:^(FIRQuerySnapshot * _Nullable snapshot,
                                    NSError * _Nullable error) {
  // ...
}];FIRSolutionAggregationViewController.m

Kotlin+KTX

db.collection("restaurants")
    .document("arinell-pizza")
    .collection("ratings")
    .get()SolutionAggregation.kt

Java

db.collection("restaurants")
        .document("arinell-pizza")
        .collection("ratings")
        .get();SolutionAggregation.java

我們不會擷取所有評分並計算匯總資訊，可以將這項資訊儲存在餐廳文件上：

網路

var arinellDoc = {
  name: 'Arinell Pizza',
  avgRating: 4.65,
  numRatings: 683
};test.solution-aggregation.js

Swift

注意：這項產品不適用於 watchOS 和 App Clip 目標。

struct Restaurant {

  let name: String
  let avgRating: Float
  let numRatings: Int

}

let arinell = Restaurant(name: "Arinell Pizza", avgRating: 4.65, numRatings: 683)SolutionAggregationViewController.swift

Objective-C

注意：這項產品不適用於 watchOS 和 App Clip 目標。

@interface FIRRestaurant : NSObject

@property (nonatomic, readonly) NSString *name;
@property (nonatomic, readonly) float averageRating;
@property (nonatomic, readonly) NSInteger ratingCount;

- (instancetype)initWithName:(NSString *)name
               averageRating:(float)averageRating
                 ratingCount:(NSInteger)ratingCount;

@end

@implementation FIRRestaurant

- (instancetype)initWithName:(NSString *)name
               averageRating:(float)averageRating
                 ratingCount:(NSInteger)ratingCount {
  self = [super init];
  if (self != nil) {
    _name = name;
    _averageRating = averageRating;
    _ratingCount = ratingCount;
  }
  return self;
}

@endFIRSolutionAggregationViewController.m

Kotlin+KTX

data class Restaurant(
    // default values required for use with "toObject"
    internal var name: String = "",
    internal var avgRating: Double = 0.0,
    internal var numRatings: Int = 0,
)

val arinell = Restaurant("Arinell Pizza", 4.65, 683)SolutionAggregation.kt

Java

public class Restaurant {
    String name;
    double avgRating;
    int numRatings;

    public Restaurant(String name, double avgRating, int numRatings) {
        this.name = name;
        this.avgRating = avgRating;
        this.numRatings = numRatings;
    }
}

Restaurant arinell = new Restaurant("Arinell Pizza", 4.65, 683);SolutionAggregation.java

為了維持這些匯總的一致性，每次都必須更新一次子集合中會新增一個評分想要維持一致性是在單一交易中執行新增和更新：

網路

function addRating(restaurantRef, rating) {
    // Create a reference for a new rating, for use inside the transaction
    var ratingRef = restaurantRef.collection('ratings').doc();

    // In a transaction, add the new rating and update the aggregate totals
    return db.runTransaction((transaction) => {
        return transaction.get(restaurantRef).then((res) => {
            if (!res.exists) {
                throw "Document does not exist!";
            }

            // Compute new number of ratings
            var newNumRatings = res.data().numRatings + 1;

            // Compute new average rating
            var oldRatingTotal = res.data().avgRating * res.data().numRatings;
            var newAvgRating = (oldRatingTotal + rating) / newNumRatings;

            // Commit to Firestore
            transaction.update(restaurantRef, {
                numRatings: newNumRatings,
                avgRating: newAvgRating
            });
            transaction.set(ratingRef, { rating: rating });
        });
    });
}test.firestore.js

Swift

注意：這項產品不適用於 watchOS 和 App Clip 目標。

func addRatingTransaction(restaurantRef: DocumentReference, rating: Float) async {
  let ratingRef: DocumentReference = restaurantRef.collection("ratings").document()

  do {
    let _ = try await db.runTransaction({ (transaction, errorPointer) -> Any? in
      do {
        let restaurantDocument = try transaction.getDocument(restaurantRef).data()
        guard var restaurantData = restaurantDocument else { return nil }

        // Compute new number of ratings
        let numRatings = restaurantData["numRatings"] as! Int
        let newNumRatings = numRatings + 1

        // Compute new average rating
        let avgRating = restaurantData["avgRating"] as! Float
        let oldRatingTotal = avgRating * Float(numRatings)
        let newAvgRating = (oldRatingTotal + rating) / Float(newNumRatings)

        // Set new restaurant info
        restaurantData["numRatings"] = newNumRatings
        restaurantData["avgRating"] = newAvgRating

        // Commit to Firestore
        transaction.setData(restaurantData, forDocument: restaurantRef)
        transaction.setData(["rating": rating], forDocument: ratingRef)
      } catch {
        // Error getting restaurant data
        // ...
      }

      return nil
    })
  } catch {
    // ...
  }
}SolutionAggregationViewController.swift

Objective-C

注意：這項產品不適用於 watchOS 和 App Clip 目標。

- (void)addRatingTransactionWithRestaurantReference:(FIRDocumentReference *)restaurant
                                             rating:(float)rating {
  FIRDocumentReference *ratingReference =
      [[restaurant collectionWithPath:@"ratings"] documentWithAutoID];

  [self.db runTransactionWithBlock:^id (FIRTransaction *transaction,
                                        NSError **errorPointer) {
    FIRDocumentSnapshot *restaurantSnapshot =
        [transaction getDocument:restaurant error:errorPointer];

    if (restaurantSnapshot == nil) {
      return nil;
    }

    NSMutableDictionary *restaurantData = [restaurantSnapshot.data mutableCopy];
    if (restaurantData == nil) {
      return nil;
    }

    // Compute new number of ratings
    NSInteger ratingCount = [restaurantData[@"numRatings"] integerValue];
    NSInteger newRatingCount = ratingCount + 1;

    // Compute new average rating
    float averageRating = [restaurantData[@"avgRating"] floatValue];
    float newAverageRating = (averageRating * ratingCount + rating) / newRatingCount;

    // Set new restaurant info

    restaurantData[@"numRatings"] = @(newRatingCount);
    restaurantData[@"avgRating"] = @(newAverageRating);

    // Commit to Firestore
    [transaction setData:restaurantData forDocument:restaurant];
    [transaction setData:@{@"rating": @(rating)} forDocument:ratingReference];
    return nil;
  } completion:^(id  _Nullable result, NSError * _Nullable error) {
    // ...
  }];
}FIRSolutionAggregationViewController.m

Kotlin+KTX

private fun addRating(restaurantRef: DocumentReference, rating: Float): Task<Void> {
    // Create reference for new rating, for use inside the transaction
    val ratingRef = restaurantRef.collection("ratings").document()

    // In a transaction, add the new rating and update the aggregate totals
    return db.runTransaction { transaction ->
        val restaurant = transaction.get(restaurantRef).toObject<Restaurant>()!!

        // Compute new number of ratings
        val newNumRatings = restaurant.numRatings + 1

        // Compute new average rating
        val oldRatingTotal = restaurant.avgRating * restaurant.numRatings
        val newAvgRating = (oldRatingTotal + rating) / newNumRatings

        // Set new restaurant info
        restaurant.numRatings = newNumRatings
        restaurant.avgRating = newAvgRating

        // Update restaurant
        transaction.set(restaurantRef, restaurant)

        // Update rating
        val data = hashMapOf<String, Any>(
            "rating" to rating,
        )
        transaction.set(ratingRef, data, SetOptions.merge())

        null
    }
}SolutionAggregation.kt

Java

private Task<Void> addRating(final DocumentReference restaurantRef, final float rating) {
    // Create reference for new rating, for use inside the transaction
    final DocumentReference ratingRef = restaurantRef.collection("ratings").document();

    // In a transaction, add the new rating and update the aggregate totals
    return db.runTransaction(new Transaction.Function<Void>() {
        @Override
        public Void apply(@NonNull Transaction transaction) throws FirebaseFirestoreException {
            Restaurant restaurant = transaction.get(restaurantRef).toObject(Restaurant.class);

            // Compute new number of ratings
            int newNumRatings = restaurant.numRatings + 1;

            // Compute new average rating
            double oldRatingTotal = restaurant.avgRating * restaurant.numRatings;
            double newAvgRating = (oldRatingTotal + rating) / newNumRatings;

            // Set new restaurant info
            restaurant.numRatings = newNumRatings;
            restaurant.avgRating = newAvgRating;

            // Update restaurant
            transaction.set(restaurantRef, restaurant);

            // Update rating
            Map<String, Object> data = new HashMap<>();
            data.put("rating", rating);
            transaction.set(ratingRef, data, SetOptions.merge());

            return null;
        }
    });
}SolutionAggregation.java

使用交易可確保您的匯總資料與基礎模型保持一致集合。如要進一步瞭解 Cloud Firestore 中的交易，請參閱交易和批次寫入。

限制

上述解決方案示範如何使用 Cloud Firestore 用戶端程式庫，但請注意下列事項限制：

安全性 - 用戶端交易需要授予用戶端權限更新資料庫中的匯總資料。雖然您可以減少這種做法的風險不一定適合各種情境
離線支援：使用者裝置會導致用戶端交易失敗處於離線狀態，也就是說，請在應用程式中處理這種情況，然後再試一次在適當的時機放送
「效能」：如果您的交易包含多次讀取、寫入與可能需要對 Pod 執行多個要求 Cloud Firestore 後端以行動裝置來說，這相當可觀
寫入費率 - 這個解決方案可能無法經常更新 Cloud Firestore 文件最多只能更新每秒搜尋一次此外，如果交易讀取的文件而在交易外部修改後，它會重試無限次結果卻失敗查看分散式計數器，瞭解需要更頻繁更新的匯總解決方案。

解決方案：使用 Cloud Functions 進行寫入時間匯總

如果用戶端交易不適合您的應用程式，您可以使用 Cloud 函式，用於更新匯總資訊每當餐廳新增評分時：

Node.js

exports.aggregateRatings = functions.firestore
    .document('restaurants/{restId}/ratings/{ratingId}')
    .onWrite(async (change, context) => {
      // Get value of the newly added rating
      const ratingVal = change.after.data().rating;

      // Get a reference to the restaurant
      const restRef = db.collection('restaurants').doc(context.params.restId);

      // Update aggregations in a transaction
      await db.runTransaction(async (transaction) => {
        const restDoc = await transaction.get(restRef);

        // Compute new number of ratings
        const newNumRatings = restDoc.data().numRatings + 1;

        // Compute new average rating
        const oldRatingTotal = restDoc.data().avgRating * restDoc.data().numRatings;
        const newAvgRating = (oldRatingTotal + ratingVal) / newNumRatings;

        // Update restaurant info
        transaction.update(restRef, {
          avgRating: newAvgRating,
          numRatings: newNumRatings
        });
      });
    });index.js

這項解決方案會將工作從用戶端卸載至代管函式，您的行動應用程式不必等待交易進行完成。在 Cloud 函式中執行的程式碼不受安全性規則約束。因此，您不再需要授予用戶端寫入匯總權限資料。

限制

使用 Cloud 函式匯總可以避免發生用戶端交易，但還是有不同的限制：

費用：您新增的每個等級都會導致發出 Cloud 函式叫用。可能會導致費用增加詳情請參閱 Cloud Functions 定價頁面。
延遲時間 - 將匯總工作卸載至 Cloud 函式後， Cloud 函式完成前，應用程式不會看到更新的資料並已通知用戶端有新資料。視乎 Cloud 函式的速度，這可能需要本機交易
寫入費率 - 這個解決方案可能無法經常更新 Cloud Firestore 文件最多只能更新每秒搜尋一次此外，如果交易讀取的文件而在交易外部修改後，它會重試無限次結果卻失敗查看分散式計數器，瞭解需要更頻繁更新的匯總解決方案。