你可以使用“IN”查询，而不是一个接一个地获取文档，你可以用一个请求获取10个文档：

• Google Firestore - How to get several documents by multiple ids in one round-trip?

userCollection.where('uid', 'in', ["1231","222","2131"]);

// or 
myCollection.where(FieldPath.documentId(), 'in', ["123","456","789"]);

// previously it was 
// myCollection.where(firestore.FieldPath.documentId(), 'in', ["123","456","789"]);

Firestore文档：“使用in运算符可以在同一字段上用逻辑OR合并最多10个相等（==）子句。in查询返回给定字段与任何比较值匹配的文档”

• https://firebase.google.com/docs/firestore/query-data/queries

当您需要一个或多个无法查询的文档时，应使用通过其标识符获取文档。不要犹豫去非规范化你的数据以使查询工作，这就是NoSQL的重点。如果我是你，我要么向这些文档添加一个字段，以便可以查询它们，要么用一个新的集合（仅用于此查询）将此数据集反规范化。但是，如果您仍然选择通过标识符获取多个文档，那么您需要发出ngetDocument请求，并使用调度组来处理它们的同步。

let docIds = ["JtThFuT4qoK4TWJGtr3n", "TL1fOBgIlX5C7qcSShGu", "UkZq3Uul5etclKepRjJF"]
let d = DispatchGroup()
    
for id in docIds {
    d.enter()
    
    Firestore.firestore().collection(...).document(id).getDocument { (document, error) in
        // handle doc
        d.leave() // always leave no matter how you exit this closure
    }
}

d.notify(queue: .main, execute: {
    // here is your completion when the last leave() was called
})

调度组所做的就是记录它进入和离开的次数，当它们匹配时，它调用它的notify(queue:execute:)方法（它的完成处理程序）。

我也面临过同样的任务。没有更好的解决方案。一个接一个地获取文档，所以我写了一个小扩展：

extension CollectionReference {
typealias MultiDocumentFetchCompletion = ([String: Result<[String: Any], Error>]) -> Void

class func fetchDocuments(with ids: [String], in collection: CollectionReference, completion:@escaping MultiDocumentFetchCompletion) -> Bool  {
    guard ids.count > 0, ids.count <= 50 else { return false }
    var results = [String: Result<[String: Any], Error>]()
    for documentId in ids {
        collection.document(documentId).getDocument(completion: { (documentSnapshot, error) in
            if let documentData = documentSnapshot?.data() {
                results[documentId] = .success(documentData)
            } else {
                results[documentId] = .failure(NSError(domain: "FIRCollectionReference", code: 0, userInfo: nil))
            }
            if results.count == ids.count {
                completion(results)
            }
        })
    }
    return true
}
}

Swift5和合并：

func getRegisteredUsers(usersId: [String]) -> AnyPublisher<[RegisteredUser], Error> {
    return Future<[RegisteredUser], Error> { promise in
        self.db.collection("registeredUsers")
            .whereField(FieldPath.documentID(), in: usersId)
            .getDocuments { snapshot, error in
                
                do {
                    let regUsers = try snapshot?.documents.compactMap {
                        try $0.data(as: RegisteredUser.self)
                    }
                    
                    promise(.success(regUsers ?? []))
                } catch {
                    promise(.failure(.default(description: error.localizedDescription)))
                }
            
        }
    }
    .eraseToAnyPublisher()
}