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# 使用 DynamoDB 中的项目
<a name="examples-dynamodb-items"></a>

在 DynamoDB 中，项目是属性的集合，每个项目都包括一个名称和一个值。******属性值可以为标量、集或文档类型。有关更多信息，请参阅《Amazon DynamoDB 开发人员指南》中的[命名规则和数据类型](https://docs.aws.amazon.com/amazondynamodb/latest/developerguide/HowItWorks.NamingRulesDataTypes.html)。

## 检索表中的项目
<a name="dynamodb-get-item"></a>

调用 [DynamoDB 客户端](https://docs.aws.amazon.com/sdk-for-cpp/latest/api/aws-cpp-sdk-dynamodb/html/class_aws_1_1_dynamo_d_b_1_1_dynamo_d_b_client.html)的 `GetItem` 方法。向它传递一个包含所需项目的表名和主键值的[GetItemRequest](https://docs.aws.amazon.com/sdk-for-cpp/latest/api/aws-cpp-sdk-dynamodb/html/class_aws_1_1_dynamo_d_b_1_1_model_1_1_get_item_request.html)对象。它返回一个[GetItemResult](https://docs.aws.amazon.com/sdk-for-cpp/latest/api/aws-cpp-sdk-dynamodb/html/class_aws_1_1_dynamo_d_b_1_1_model_1_1_get_item_result.html)对象。

您可以使用返回`GetItemResult`对象的`GetItem()`方法来检索与该项目关联`Aws::Map`的键`Aws::String`和值[AttributeValue](https://docs.aws.amazon.com/sdk-for-cpp/latest/api/aws-cpp-sdk-dynamodb/html/class_aws_1_1_dynamo_d_b_1_1_model_1_1_attribute_value.html)对。

 **包含** 

```
#include <aws/core/Aws.h>
#include <aws/dynamodb/DynamoDBClient.h>
#include <aws/dynamodb/model/AttributeDefinition.h>
#include <aws/dynamodb/model/GetItemRequest.h>
#include <iostream>
```

 **代码** 

```
//! Get an item from an Amazon DynamoDB table.
/*!
  \sa getItem()
  \param tableName: The table name.
  \param partitionKey: The partition key.
  \param partitionValue: The value for the partition key.
  \param clientConfiguration: AWS client configuration.
  \return bool: Function succeeded.
 */

bool AwsDoc::DynamoDB::getItem(const Aws::String &tableName,
                               const Aws::String &partitionKey,
                               const Aws::String &partitionValue,
                               const Aws::Client::ClientConfiguration &clientConfiguration) {
    Aws::DynamoDB::DynamoDBClient dynamoClient(clientConfiguration);
    Aws::DynamoDB::Model::GetItemRequest request;

    // Set up the request.
    request.SetTableName(tableName);
    request.AddKey(partitionKey,
                   Aws::DynamoDB::Model::AttributeValue().SetS(partitionValue));

    // Retrieve the item's fields and values.
    const Aws::DynamoDB::Model::GetItemOutcome &outcome = dynamoClient.GetItem(request);
    if (outcome.IsSuccess()) {
        // Reference the retrieved fields/values.
        const Aws::Map<Aws::String, Aws::DynamoDB::Model::AttributeValue> &item = outcome.GetResult().GetItem();
        if (!item.empty()) {
            // Output each retrieved field and its value.
            for (const auto &i: item)
                std::cout << "Values: " << i.first << ": " << i.second.GetS()
                          << std::endl;
        }
        else {
            std::cout << "No item found with the key " << partitionKey << std::endl;
        }
    }
    else {
        std::cerr << "Failed to get item: " << outcome.GetError().GetMessage();
    }

    return outcome.IsSuccess();
}
```

请参阅上的[完整示例](https://github.com/awsdocs/aws-doc-sdk-examples/tree/main/cpp/example_code/dynamodb/get_item.cpp) GitHub。

## 向表中添加项目
<a name="dynamodb-add-item"></a>

创建代表每个项目的键`Aws::String`和值[AttributeValue](https://docs.aws.amazon.com/sdk-for-cpp/latest/api/aws-cpp-sdk-dynamodb/html/class_aws_1_1_dynamo_d_b_1_1_model_1_1_attribute_value.html)对。其中必须包括表的主键字段的值。如果主键标识的项目已存在，那么其字段将通过该请求*更新*。[PutItemRequest](https://docs.aws.amazon.com/sdk-for-cpp/latest/api/aws-cpp-sdk-dynamodb/html/class_aws_1_1_dynamo_d_b_1_1_model_1_1_put_item_request.html)使用`AddItem`方法将它们添加到中。

 **包含** 

```
#include <aws/core/Aws.h>
#include <aws/dynamodb/DynamoDBClient.h>
#include <aws/dynamodb/model/AttributeDefinition.h>
#include <aws/dynamodb/model/PutItemRequest.h>
#include <aws/dynamodb/model/PutItemResult.h>
#include <iostream>
```

 **代码** 

```
//! Put an item in an Amazon DynamoDB table.
/*!
  \sa putItem()
  \param tableName: The table name.
  \param artistKey: The artist key. This is the partition key for the table.
  \param artistValue: The artist value.
  \param albumTitleKey: The album title key.
  \param albumTitleValue: The album title value.
  \param awardsKey: The awards key.
  \param awardsValue: The awards value.
  \param songTitleKey: The song title key.
  \param songTitleValue: The song title value.
  \param clientConfiguration: AWS client configuration.
  \return bool: Function succeeded.
 */
bool AwsDoc::DynamoDB::putItem(const Aws::String &tableName,
                               const Aws::String &artistKey,
                               const Aws::String &artistValue,
                               const Aws::String &albumTitleKey,
                               const Aws::String &albumTitleValue,
                               const Aws::String &awardsKey,
                               const Aws::String &awardsValue,
                               const Aws::String &songTitleKey,
                               const Aws::String &songTitleValue,
                               const Aws::Client::ClientConfiguration &clientConfiguration) {
    Aws::DynamoDB::DynamoDBClient dynamoClient(clientConfiguration);

    Aws::DynamoDB::Model::PutItemRequest putItemRequest;
    putItemRequest.SetTableName(tableName);

    putItemRequest.AddItem(artistKey, Aws::DynamoDB::Model::AttributeValue().SetS(
            artistValue)); // This is the hash key.
    putItemRequest.AddItem(albumTitleKey, Aws::DynamoDB::Model::AttributeValue().SetS(
            albumTitleValue));
    putItemRequest.AddItem(awardsKey,
                           Aws::DynamoDB::Model::AttributeValue().SetS(awardsValue));
    putItemRequest.AddItem(songTitleKey,
                           Aws::DynamoDB::Model::AttributeValue().SetS(songTitleValue));

    const Aws::DynamoDB::Model::PutItemOutcome outcome = dynamoClient.PutItem(
            putItemRequest);
    if (outcome.IsSuccess()) {
        std::cout << "Successfully added Item!" << std::endl;
    }
    else {
        std::cerr << outcome.GetError().GetMessage() << std::endl;
        return false;
    }

    return waitTableActive(tableName, dynamoClient);
}
```

请参阅上的[完整示例](https://github.com/awsdocs/aws-doc-sdk-examples/tree/main/cpp/example_code/dynamodb/put_item.cpp) GitHub。

## 更新表中现有项目
<a name="dynamodb-update-item"></a>

您可以使用 Dynamo 的`UpdateItem`方法更新表中已存在DBClient的项目的属性，提供表名、主键值和要更新的字段及其对应的值。

 **导入** 

```
#include <aws/core/Aws.h>
#include <aws/dynamodb/DynamoDBClient.h>
#include <aws/dynamodb/model/UpdateItemRequest.h>
#include <aws/dynamodb/model/UpdateItemResult.h>
#include <iostream>
```

 **代码** 

```
//! Update an Amazon DynamoDB table item.
/*!
  \sa updateItem()
  \param tableName: The table name.
  \param partitionKey: The partition key.
  \param partitionValue: The value for the partition key.
  \param attributeKey: The key for the attribute to be updated.
  \param attributeValue: The value for the attribute to be updated.
  \param clientConfiguration: AWS client configuration.
  \return bool: Function succeeded.
  */

/*
 *  The example code only sets/updates an attribute value. It processes
 *  the attribute value as a string, even if the value could be interpreted
 *  as a number. Also, the example code does not remove an existing attribute
 *  from the key value.
 */

bool AwsDoc::DynamoDB::updateItem(const Aws::String &tableName,
                                  const Aws::String &partitionKey,
                                  const Aws::String &partitionValue,
                                  const Aws::String &attributeKey,
                                  const Aws::String &attributeValue,
                                  const Aws::Client::ClientConfiguration &clientConfiguration) {
    Aws::DynamoDB::DynamoDBClient dynamoClient(clientConfiguration);

    // *** Define UpdateItem request arguments.
    // Define TableName argument.
    Aws::DynamoDB::Model::UpdateItemRequest request;
    request.SetTableName(tableName);

    // Define KeyName argument.
    Aws::DynamoDB::Model::AttributeValue attribValue;
    attribValue.SetS(partitionValue);
    request.AddKey(partitionKey, attribValue);

    // Construct the SET update expression argument.
    Aws::String update_expression("SET #a = :valueA");
    request.SetUpdateExpression(update_expression);

    // Construct attribute name argument.
    Aws::Map<Aws::String, Aws::String> expressionAttributeNames;
    expressionAttributeNames["#a"] = attributeKey;
    request.SetExpressionAttributeNames(expressionAttributeNames);

    // Construct attribute value argument.
    Aws::DynamoDB::Model::AttributeValue attributeUpdatedValue;
    attributeUpdatedValue.SetS(attributeValue);
    Aws::Map<Aws::String, Aws::DynamoDB::Model::AttributeValue> expressionAttributeValues;
    expressionAttributeValues[":valueA"] = attributeUpdatedValue;
    request.SetExpressionAttributeValues(expressionAttributeValues);

    // Update the item.
    const Aws::DynamoDB::Model::UpdateItemOutcome &outcome = dynamoClient.UpdateItem(
            request);
    if (outcome.IsSuccess()) {
        std::cout << "Item was updated" << std::endl;
    } else {
        std::cerr << outcome.GetError().GetMessage() << std::endl;
        return false;
    }

    return waitTableActive(tableName, dynamoClient);
}
```

请参阅[完整示例](https://github.com/awsdocs/aws-doc-sdk-examples/tree/main/cpp/example_code/dynamodb/update_item.cpp)。

## 更多信息
<a name="more-info"></a>
+  《Amazon DynamoDB 开发人员指南》中的[项目处理准则](https://docs.aws.amazon.com/amazondynamodb/latest/developerguide/GuidelinesForItems.html)
+  《Amazon DynamoDB 开发人员指南》中的[处理 DynamoDB 中的项目](https://docs.aws.amazon.com/amazondynamodb/latest/developerguide/WorkingWithItems.html)