c# - 使用 Microsoft.Azure.Cosmos 的 FeedResponse 时如何获取单个文档的 ETag

Question

我正在查看来自 https://github.com/Azure/azure-cosmos-dotnet-v3/blob/dc3468bd5ce828e504ddef92ef792c35370de055/Microsoft.Azure.Cosmos.Samples/Usage/ItemManagement/Program.cs#L590 的样本

下面是使用container.ReadItemAsync时使用ETag的示例:

            ItemResponse<SalesOrder> itemResponse = await container.ReadItemAsync<SalesOrder>(
                partitionKey: new PartitionKey("Account1"),
                id: "SalesOrder1");

            Console.WriteLine("ETag of read item - {0}", itemResponse.ETag);

            SalesOrder item = itemResponse;
            //Update the total due
            itemResponse.Resource.TotalDue = 1000000;

            //persist the change back to the server
            ItemResponse<SalesOrder> updatedDoc = await container.ReplaceItemAsync<SalesOrder>(
                partitionKey: new PartitionKey(item.AccountNumber),
                id: item.Id,
                item: item);

            Console.WriteLine("ETag of item now that is has been updated - {0}", updatedDoc.ETag);

            //now, using the originally retrieved item do another update 
            //but set the AccessCondition class with the ETag of the originally read item and also set the AccessConditionType
            //this tells the service to only do this operation if ETag on the request matches the current ETag on the item
            //in our case it won't, because we updated the item and therefore gave it a new ETag
            try
            {
                itemResponse.Resource.TotalDue = 9999999;
                updatedDoc = await container.ReplaceItemAsync<SalesOrder>(itemResponse, item.Id, new PartitionKey(item.AccountNumber), new ItemRequestOptions { IfMatchEtag = itemResponse.ETag });
            }
            catch (CosmosException cre)
            {
                //   now notice the failure when attempting the update 
                //   this is because the ETag on the server no longer matches the ETag of doc (b/c it was changed in step 2)
                if (cre.StatusCode == HttpStatusCode.PreconditionFailed)
                {
                    Console.WriteLine("As expected, we have a pre-condition failure exception\n");
                }
            }

在我的场景中，我有一个查询，我需要获取 ETag 以便稍后在更新中使用。在此示例中，我们得到 FeedResponse<SalesOrder>其中有 ETag，但这是与最后一笔交易相关的标签，而不是文档。所以我的问题是如何获取单个文档的 ETag？

        private static async Task QueryItems()
        {
            //******************************************************************************************************************
            // 1.4 - Query for items by a property other than Id
            //
            // NOTE: Operations like AsEnumerable(), ToList(), ToArray() will make as many trips to the database
            //       as required to fetch the entire result-set. Even if you set MaxItemCount to a smaller number. 
            //       MaxItemCount just controls how many results to fetch each trip. 
            //******************************************************************************************************************
            Console.WriteLine("\n1.4 - Querying for a item using its AccountNumber property");

            QueryDefinition query = new QueryDefinition(
                "select * from sales s where s.AccountNumber = @AccountInput ")
                .WithParameter("@AccountInput", "Account1");

            FeedIterator<SalesOrder> resultSet = container.GetItemQueryIterator<SalesOrder>(
                query,
                requestOptions: new QueryRequestOptions()
                {
                    PartitionKey = new PartitionKey("Account1"),
                    MaxItemCount = 1
                });

            List<SalesOrder> allSalesForAccount1 = new List<SalesOrder>();
            while (resultSet.HasMoreResults)
            {
                FeedResponse<SalesOrder> response = await resultSet.ReadNextAsync();
                SalesOrder sale = response.First();
                Console.WriteLine($"\n1.4.1 Account Number: {sale.AccountNumber}; Id: {sale.Id};");
                if(response.Diagnostics != null)
                {
                    Console.WriteLine($" Diagnostics {response.Diagnostics.ToString()}");
                }

                allSalesForAccount1.Add(sale);
            }

Answer 1

执行查询时，FeedResponse 包含 T 类型的对象列表。如果 T 包含与 _etag 系统属性匹配的属性，则可以获得 ETag。 Source Github issue comment .

例如，使用 NewtonSoft.JSON 的 JsonProperty 属性，我必须将以下属性添加到我的类型 T

[JsonProperty("_etag")]
public string ETag { get; set; }