本文為英文版的機器翻譯版本，如內容有任何歧義或不一致之處，概以英文版為準。

# 避免儲存巢狀物件的 null 屬性
<a name="ddb-en-client-adv-features-ignore-null"></a>

將資料類別物件儲存至 DynamoDB 時，您可以透過套用`@DynamoDbIgnoreNulls`註釋來略過巢狀物件的 null 屬性。相反地，具有 null 值的頂層屬性絕不會儲存到資料庫。

為了說明註釋的運作方式，程式碼範例會使用下列兩個 Bean。

## 範例 Bean
<a name="ddb-en-client-adv-features-ignore-null-ex1"></a>

下列資料類別包含兩個`InnerBean`欄位。不會註釋 getter 方法 `getInnerBeanWithoutAnno()`。`getInnerBeanWithIgnoreNullsAnno()` 方法會以 標註`@DynamoDbIgnoreNulls`。

```
@DynamoDbBean
public class MyBean {

    private String id;
    private String name;
    private InnerBean innerBeanWithoutAnno;
    private InnerBean innerBeanWithIgnoreNullsAnno;

    @DynamoDbPartitionKey
    public String getId() { return id; }
    public void setId(String id) { this.id = id; }

    public String getName() { return name; }
    public void setName(String name) { this.name = name; }

    public InnerBean getInnerBeanWithoutAnno() { return innerBeanWithoutAnno; }
    public void setInnerBeanWithoutAnno(InnerBean innerBeanWithoutAnno) { this.innerBeanWithoutAnno = innerBeanWithoutAnno; }

    @DynamoDbIgnoreNulls
    public InnerBean getInnerBeanWithIgnoreNullsAnno() { return innerBeanWithIgnoreNullsAnno; }
    public void setInnerBeanWithIgnoreNullsAnno(InnerBean innerBeanWithAnno) { this.innerBeanWithIgnoreNullsAnno = innerBeanWithAnno; }

    @Override
    public String toString() {
        return new StringJoiner(", ", MyBean.class.getSimpleName() + "[", "]")
                .add("innerBeanWithoutAnno=" + innerBeanWithoutAnno)
                .add("innerBeanWithIgnoreNullsAnno=" + innerBeanWithIgnoreNullsAnno)
                .add("id='" + id + "'")
                .add("name='" + name + "'")
                .toString();
    }
}
```

下列`InnerBean`類別的執行個體是 的欄位，`MyBean`並用於下列範例程式碼。

```
@DynamoDbBean
public class InnerBean {

    private String innerBeanFieldString;
    private Integer innerBeanFieldInteger;

    public String getInnerBeanFieldString() { return innerBeanFieldString; }
    public void setInnerBeanFieldString(String innerBeanFieldString) { this.innerBeanFieldString = innerBeanFieldString; }

    public Integer getInnerBeanFieldInteger() { return innerBeanFieldInteger; }
    public void setInnerBeanFieldInteger(Integer innerBeanFieldInteger) { this.innerBeanFieldInteger = innerBeanFieldInteger; }

    @Override
    public String toString() {
        return new StringJoiner(", ", InnerBean.class.getSimpleName() + "[", "]")
                .add("innerBeanFieldString='" + innerBeanFieldString + "'")
                .add("innerBeanFieldInteger=" + innerBeanFieldInteger)
                .toString();
    }
}
```

下列程式碼範例會建立 `InnerBean` 物件，並使用 值僅設定其兩個屬性的其中之一。

```
    public void ignoreNullsAnnoUsingPutItemExample(DynamoDbTable<MyBean> myBeanTable) {
        // Create an InnerBean object and give only one attribute a value.
        InnerBean innerBeanOneAttributeSet = new InnerBean();
        innerBeanOneAttributeSet.setInnerBeanFieldInteger(200);

        // Create a MyBean instance and use the same InnerBean instance both for attributes.
        MyBean bean = new MyBean();
        bean.setId("1");
        bean.setInnerBeanWithoutAnno(innerBeanOneAttributeSet);
        bean.setInnerBeanWithIgnoreNullsAnno(innerBeanOneAttributeSet);

        Map<String, AttributeValue> itemMap = myBeanTable.tableSchema().itemToMap(bean, true);
        logger.info(itemMap.toString());
        // Log the map that is sent to the database.
        // {innerBeanWithIgnoreNullsAnno=AttributeValue(M={innerBeanFieldInteger=AttributeValue(N=200)}), id=AttributeValue(S=1), innerBeanWithoutAnno=AttributeValue(M={innerBeanFieldInteger=AttributeValue(N=200), innerBeanFieldString=AttributeValue(NUL=true)})}
        
        // Save the MyBean object to the table.
        myBeanTable.putItem(bean);
    }
```

若要視覺化傳送至 DynamoDB 的低階資料，程式碼會在儲存`MyBean`物件之前記錄屬性映射。

記錄的輸出顯示`innerBeanWithIgnoreNullsAnno`輸出一個屬性，

```
innerBeanWithIgnoreNullsAnno=AttributeValue(M={innerBeanFieldInteger=AttributeValue(N=200)})
```

`innerBeanWithoutAnno` 執行個體會輸出兩個屬性。一個屬性的值為 200，另一個是 null 值的屬性。

```
innerBeanWithoutAnno=AttributeValue(M={innerBeanFieldInteger=AttributeValue(N=200), innerBeanFieldString=AttributeValue(NUL=true)})
```

## 屬性映射的 JSON 表示法
<a name="ddb-en-client-adv-features-ignore-null-ex2"></a>

下列 JSON 表示法可讓您更輕鬆地查看儲存至 DynamoDB 的資料。

```
{
  "id": {
    "S": "1"
  },
  "innerBeanWithIgnoreNullsAnno": {
    "M": {
      "innerBeanFieldInteger": {
        "N": "200"
      }
    }
  },
  "innerBeanWithoutAnno": {
    "M": {
      "innerBeanFieldInteger": {
        "N": "200"
      },
      "innerBeanFieldString": {
        "NULL": true
      }
    }
  }
}
```

## 替代靜態結構描述
<a name="ddb-en-client-adv-features-empty-ex1-static"></a>

您可以使用下列`StaticTableSchema`版本的資料表結構描述來放置資料類別註釋。

```
public static TableSchema<MyBean> buildStaticSchemas() {

    StaticTableSchema<InnerBean> innerBeanStaticTableSchema =
        StaticTableSchema.builder(InnerBean.class)
            .newItemSupplier(InnerBean::new)
            .addAttribute(String.class, a -> a.name("innerBeanFieldString")
                .getter(InnerBean::getInnerBeanFieldString)
                .setter(InnerBean::setInnerBeanFieldString))
            .addAttribute(Integer.class, a -> a.name("innerBeanFieldInteger")
                .getter(InnerBean::getInnerBeanFieldInteger)
                .setter(InnerBean::setInnerBeanFieldInteger))
            .build();

    return StaticTableSchema.builder(MyBean.class)
        .newItemSupplier(MyBean::new)
        .addAttribute(String.class, a -> a.name("id")
            .getter(MyBean::getId)
            .setter(MyBean::setId)
            .addTag(primaryPartitionKey()))
        .addAttribute(String.class, a -> a.name("name")
            .getter(MyBean::getName)
            .setter(MyBean::setName))
        .addAttribute(EnhancedType.documentOf(InnerBean.class,
                innerBeanStaticTableSchema),
            a -> a.name("innerBeanWithoutAnno")
                .getter(MyBean::getInnerBeanWithoutAnno)
                .setter(MyBean::setInnerBeanWithoutAnno))
        .addAttribute(EnhancedType.documentOf(InnerBean.class,
                innerBeanStaticTableSchema,
                b -> b.ignoreNulls(true)),
            a -> a.name("innerBeanWithIgnoreNullsAnno")
                .getter(MyBean::getInnerBeanWithIgnoreNullsAnno)
                .setter(MyBean::setInnerBeanWithIgnoreNullsAnno))
        .build();
}
```