处理嵌套属性 - AWS SDK for Java 2.x
映射嵌套属性投影嵌套属性

本文属于机器翻译版本。若本译文内容与英语原文存在差异，则一律以英文原文为准。

处理嵌套属性

DynamoDB 中的嵌套属性嵌入在另一个属性中，例如列表元素和映射条目。

在 Java 中，DynamoDB 嵌套属性对应于 List 或 Map 类的成员。它还对应于复杂类型的实例，例如 Address 或 PhoneNumber，如用于以下 Person 类中。

@DynamoDbBean
public class Person {
    Integer id;
    String firstName;
    String lastName;
    Integer age;
    Map<String, Address> addresses;
    List<PhoneNumber> phoneNumbers;

    List<String> hobbies;

    @DynamoDbPartitionKey
    public Integer getId() {
        return id;
    }

    public void setId(Integer id) {
        this.id = id;
    }

    public String getFirstName() {
        return firstName;
    }

    public void setFirstName(String firstName) {
        this.firstName = firstName;
    }

    public String getLastName() {
        return lastName;
    }

    public void setLastName(String lastName) {
        this.lastName = lastName;
    }

    public Integer getAge() {
        return age;
    }

    public void setAge(Integer age) {
        this.age = age;
    }

    public Map<String, Address> getAddresses() {
        return addresses;
    }

    public void setAddresses(Map<String, Address> addresses) {
        this.addresses = addresses;
    }

    public List<PhoneNumber> getPhoneNumbers() {
        return phoneNumbers;
    }

    public void setPhoneNumbers(List<PhoneNumber> phoneNumbers) {
        this.phoneNumbers = phoneNumbers;
    }

    public List<String> getHobbies() {
        return hobbies;
    }

    public void setHobbies(List<String> hobbies) {
        this.hobbies = hobbies;
    }

    @Override
    public String toString() {
        return "Person{" +
                "id=" + id +
                ", firstName='" + firstName + '\'' +
                ", lastName='" + lastName + '\'' +
                ", age=" + age +
                ", addresses=" + addresses +
                ", phoneNumbers=" + phoneNumbers +
                ", hobbies=" + hobbies +
                '}';
    }
}
@DynamoDbBean
public class Address {
    private String street;
    private String city;
    private String state;
    private String zipCode;

    public Address() {
    }

    public String getStreet() {
        return this.street;
    }

    public String getCity() {
        return this.city;
    }

    public String getState() {
        return this.state;
    }

    public String getZipCode() {
        return this.zipCode;
    }

    public void setStreet(String street) {
        this.street = street;
    }

    public void setCity(String city) {
        this.city = city;
    }

    public void setState(String state) {
        this.state = state;
    }

    public void setZipCode(String zipCode) {
        this.zipCode = zipCode;
    }

    @Override
    public boolean equals(Object o) {
        if (this == o) return true;
        if (o == null || getClass() != o.getClass()) return false;
        Address address = (Address) o;
        return Objects.equals(street, address.street) && Objects.equals(city, address.city) && Objects.equals(state, address.state) && Objects.equals(zipCode, address.zipCode);
    }

    @Override
    public int hashCode() {
        return Objects.hash(street, city, state, zipCode);
    }

    @Override
    public String toString() {
        return "Address{" +
                "street='" + street + '\'' +
                ", city='" + city + '\'' +
                ", state='" + state + '\'' +
                ", zipCode='" + zipCode + '\'' +
                '}';
    }
}
@DynamoDbBean
public class PhoneNumber {
    String type;
    String number;

    public String getType() {
        return type;
    }

    public void setType(String type) {
        this.type = type;
    }

    public String getNumber() {
        return number;
    }

    public void setNumber(String number) {
        this.number = number;
    }

    @Override
    public String toString() {
        return "PhoneNumber{" +
                "type='" + type + '\'' +
                ", number='" + number + '\'' +
                '}';
    }
}

映射嵌套属性

使用带注释的类

您可以通过注释来保存自定义类的嵌套属性。前面显示的 Address 类和 PhoneNumber 类仅使用 @DynamoDbBean 注释进行注释。当 DynamoDB 增强型客户端 API 使用以下代码段为 Person 类生成表架构时，API 会发现 Address 和 PhoneNumber 类的使用，并生成相应的映射以与 DynamoDB 结合使用。

TableSchema<Person> personTableSchema = TableSchema.fromBean(Person.class);

使用嵌套架构

另一种方法是为每个类使用静态表架构生成器，如以下代码所示。

Address 和 PhoneNumber 类的表架构是抽象的，不能与 DynamoDB 表一起使用。这是因为它们缺少主键的定义。但是，它们在 Person 类的表架构中用作嵌套架构。

在注释行 1 和 2 之后的 PERSON_TABLE_SCHEMA 定义中，显示了使用抽象表架构的代码。在 EnhanceType.documentOf(...) 方法中使用 documentOf 并不表示该方法将返回增强型文档 API 的 EnhancedDocument 类型。在此上下文中，documentOf(...) 方法将返回一个对象，该对象知道如何使用表架构参数，在其类参数和 DynamoDB 表属性之间进行映射。

    // Abstract table schema that cannot be used to work with a DynamoDB table,
    // but can be used as a nested schema.
    public static final TableSchema<Address> TABLE_SCHEMA_ADDRESS = TableSchema.builder(Address.class)
            .newItemSupplier(Address::new)
            .addAttribute(String.class, a -> a.name("street")
                    .getter(Address::getStreet)
                    .setter(Address::setStreet))
            .addAttribute(String.class, a -> a.name("city")
                    .getter(Address::getCity)
                    .setter(Address::setCity))
            .addAttribute(String.class, a -> a.name("zipcode")
                    .getter(Address::getZipCode)
                    .setter(Address::setZipCode))
            .addAttribute(String.class, a -> a.name("state")
                    .getter(Address::getState)
                    .setter(Address::setState))
            .build();

    // Abstract table schema that cannot be used to work with a DynamoDB table,
    // but can be used as a nested schema.
    public static final TableSchema<PhoneNumber> TABLE_SCHEMA_PHONENUMBER = TableSchema.builder(PhoneNumber.class)
            .newItemSupplier(PhoneNumber::new)
            .addAttribute(String.class, a -> a.name("type")
                    .getter(PhoneNumber::getType)
                    .setter(PhoneNumber::setType))
            .addAttribute(String.class, a -> a.name("number")
                    .getter(PhoneNumber::getNumber)
                    .setter(PhoneNumber::setNumber))
            .build();

    // A static table schema that can be used with a DynamoDB table.
    // The table schema contains two nested schemas that are used to perform mapping to/from DynamoDB.
    public static final TableSchema<Person> PERSON_TABLE_SCHEMA =
            TableSchema.builder(Person.class)
                    .newItemSupplier(Person::new)
                    .addAttribute(Integer.class, a -> a.name("id")
                            .getter(Person::getId)
                            .setter(Person::setId)
                            .addTag(StaticAttributeTags.primaryPartitionKey()))
                    .addAttribute(String.class, a -> a.name("firstName")
                            .getter(Person::getFirstName)
                            .setter(Person::setFirstName))
                    .addAttribute(String.class, a -> a.name("lastName")
                            .getter(Person::getLastName)
                            .setter(Person::setLastName))
                    .addAttribute(Integer.class, a -> a.name("age")
                            .getter(Person::getAge)
                            .setter(Person::setAge))
                    .addAttribute(EnhancedType.listOf(String.class), a -> a.name("hobbies")
                            .getter(Person::getHobbies)
                            .setter(Person::setHobbies))
                    .addAttribute(EnhancedType.mapOf(
                            EnhancedType.of(String.class),
                            // 1. Use mapping functionality of the Address table schema.
                            EnhancedType.documentOf(Address.class, TABLE_SCHEMA_ADDRESS)), a -> a.name("addresses")
                            .getter(Person::getAddresses)
                            .setter(Person::setAddresses))
                    .addAttribute(EnhancedType.listOf(
                            // 2. Use mapping functionality of the PhoneNumber table schema.
                            EnhancedType.documentOf(PhoneNumber.class, TABLE_SCHEMA_PHONENUMBER)), a -> a.name("phoneNumbers")
                            .getter(Person::getPhoneNumbers)
                            .setter(Person::setPhoneNumbers))
                    .build();

投影嵌套属性

对于 query() 和 scan() 方法，您可以使用 addNestedAttributeToProject() 和 attributesToProject() 之类的方法调用来指定要在结果中返回哪些属性。在发送请求之前，DynamoDB 增强型客户端 API 会将 Java 方法调用参数转换为投影表达式。

以下示例在 Person 表中填充两个项目，然后执行三个扫描操作。

第一个扫描访问表中的所有项目，以便将结果与其他扫描操作进行比较。

第二个扫描使用 addNestedAttributeToProject() 生成器方法仅返回 street 属性值。

第三个扫描操作使用 attributesToProject() 生成器方法返回第一级属性 hobbies 的数据。hobbies 的属性类型是列表。要访问单个列表项目，请对列表执行 get() 操作。

        personDynamoDbTable = getDynamoDbEnhancedClient().table("Person", PERSON_TABLE_SCHEMA);
        PersonUtils.createPersonTable(personDynamoDbTable, getDynamoDbClient());
        // Use a utility class to add items to the Person table.
        List<Person> personList = PersonUtils.getItemsForCount(2);
        // This utility method performs a put against DynamoDB to save the instances in the list argument.
        PersonUtils.putCollection(getDynamoDbEnhancedClient(), personList, personDynamoDbTable);

        // The first scan logs all items in the table to compare to the results of the subsequent scans.
        final PageIterable<Person> allItems = personDynamoDbTable.scan();
        allItems.items().forEach(p ->
                // 1. Log what is in the table.
                logger.info(p.toString()));

        // Scan for nested attributes.
        PageIterable<Person> streetScanResult = personDynamoDbTable.scan(b -> b
                // Use the 'addNestedAttributeToProject()' or 'addNestedAttributesToProject()' to access data nested in maps in DynamoDB.
                .addNestedAttributeToProject(
                        NestedAttributeName.create("addresses", "work", "street")
                ));

        streetScanResult.items().forEach(p ->
                //2. Log the results of requesting nested attributes.
                logger.info(p.toString()));

        // Scan for a top-level list attribute.
        PageIterable<Person> phoneNumbersScanResult = personDynamoDbTable.scan(b -> b
                // Use the 'attributesToProject()' method to access first-level attributes.
                .attributesToProject("hobbies"));

        phoneNumbersScanResult.items().forEach((p) -> {
            // 3. Log the results of the request for the 'hobbies' attribute.
            logger.info(p.toString());
            // To access an item in a list, first get the parent attribute, 'hobbies', then access items in the list.
            String hobby = p.getHobbies().get(1);
            // 4. Log an item in the list.
            logger.info(hobby);
        });

// Logged results from comment line 1.
Person{id=2, firstName='first name 2', lastName='last name 2', age=11, addresses={work=Address{street='street 21', city='city 21', state='state 21', zipCode='33333'}, home=Address{street='street 2', city='city 2', state='state 2', zipCode='22222'}}, phoneNumbers=[PhoneNumber{type='home', number='222-222-2222'}, PhoneNumber{type='work', number='333-333-3333'}], hobbies=[hobby 2, hobby 21]}
Person{id=1, firstName='first name 1', lastName='last name 1', age=11, addresses={work=Address{street='street 11', city='city 11', state='state 11', zipCode='22222'}, home=Address{street='street 1', city='city 1', state='state 1', zipCode='11111'}}, phoneNumbers=[PhoneNumber{type='home', number='111-111-1111'}, PhoneNumber{type='work', number='222-222-2222'}], hobbies=[hobby 1, hobby 11]}

// Logged results from comment line 2.
Person{id=null, firstName='null', lastName='null', age=null, addresses={work=Address{street='street 21', city='null', state='null', zipCode='null'}}, phoneNumbers=null, hobbies=null}
Person{id=null, firstName='null', lastName='null', age=null, addresses={work=Address{street='street 11', city='null', state='null', zipCode='null'}}, phoneNumbers=null, hobbies=null}

// Logged results from comment lines 3 and 4.
Person{id=null, firstName='null', lastName='null', age=null, addresses=null, phoneNumbers=null, hobbies=[hobby 2, hobby 21]}
hobby 21
Person{id=null, firstName='null', lastName='null', age=null, addresses=null, phoneNumbers=null, hobbies=[hobby 1, hobby 11]}
hobby 11
注意

如果 attributesToProject() 方法遵循任何其他用于添加要投影的属性的生成器方法，则提供给 attributesToProject() 的属性名称列表将替换所有其他属性名称。

在以下代码段中，使用 ScanEnhancedRequest 实例执行的扫描仅返回业余爱好数据。

ScanEnhancedRequest lastOverwrites = ScanEnhancedRequest.builder()
        .addNestedAttributeToProject(
                NestedAttributeName.create("addresses", "work", "street"))
        .addAttributeToProject("firstName")
        // If the 'attributesToProject()' method follows other builder methods that add attributes for projection,
        // its list of attributes replace all previous attributes.
        .attributesToProject("hobbies")
        .build();
PageIterable<Person> hobbiesOnlyResult = personDynamoDbTable.scan(lastOverwrites);
hobbiesOnlyResult.items().forEach(p ->
        logger.info(p.toString()));

// Logged results.
Person{id=null, firstName='null', lastName='null', age=null, addresses=null, phoneNumbers=null, hobbies=[hobby 2, hobby 21]}
Person{id=null, firstName='null', lastName='null', age=null, addresses=null, phoneNumbers=null, hobbies=[hobby 1, hobby 11]}

以下代码段首先使用 attributesToProject() 方法。此排序保留了请求的所有其他属性。

ScanEnhancedRequest attributesPreserved = ScanEnhancedRequest.builder()
        // Use 'attributesToProject()' first so that the method call does not replace all other attributes
        // that you want to project.
        .attributesToProject("firstName")
        .addNestedAttributeToProject(
                NestedAttributeName.create("addresses", "work", "street"))
        .addAttributeToProject("hobbies")
        .build();
PageIterable<Person> allAttributesResult = personDynamoDbTable.scan(attributesPreserved);
allAttributesResult.items().forEach(p ->
        logger.info(p.toString()));

// Logged results.
Person{id=null, firstName='first name 2', lastName='null', age=null, addresses={work=Address{street='street 21', city='null', state='null', zipCode='null'}}, phoneNumbers=null, hobbies=[hobby 2, hobby 21]}
Person{id=null, firstName='first name 1', lastName='null', age=null, addresses={work=Address{street='street 11', city='null', state='null', zipCode='null'}}, phoneNumbers=null, hobbies=[hobby 1, hobby 11]}