aws-sdk-java-v2 Support optional prefix for @DynamoDbFlatten fields

Describe the feature

In order to provide better clarity to field names in Flattened DynamoDdBeans we need to be able to prefix field names with additional context. I propose adding an optional String prefix to the DynamoDbFlatten annotation that would allow users to supply a prefix that would be appended to flatten field names in the parent object. Leaving the property unset would result in the current behavior of no prefixing in order to maintain backwards compatability.

Use Case

I want to flatten multiple objects of the same type but with different meanings and not have their names conflict.

// Record.java

import java.time.Instant;

import lombok.Data;
import lombok.Getter;
import software.amazon.awssdk.enhanced.dynamodb.mapper.annotations.DynamoDbBean;
import software.amazon.awssdk.enhanced.dynamodb.mapper.annotations.DynamoDbFlatten;
import software.amazon.awssdk.enhanced.dynamodb.mapper.annotations.DynamoDbPartitionKey;

@Data
@DynamoDbBean
public class Record {
    @Getter(onMethod_ = {@DynamoDbPartitionKey})
    String id;
    @Getter(onMethod_ = {@DynamoDbFlatten})
    Edit created;
    @Getter(onMethod_ = {@DynamoDbFlatten})
    Edit updated;

    @Data
    @DynamoDbBean
    public static class Edit {
        String id;
        Instant timestamp;
    }
}

Proposed Solution

Extend the existing FlattenedMapper class to include an optional prefix that would be appended to each attribute when building the map of mappers.

public @interface DynamoDbFlatten {
    String prefix default "";
}

DynamoDbFlatten dynamoDbFlatten = getPropertyAnnotation(propertyDescriptor, DynamoDbFlatten.class);

if (dynamoDbFlatten != null) {
  builder.flatten(TableSchema.fromClass(propertyDescriptor.getter().getReturnType()),
                  getterForProperty(propertyDescriptor, immutableClass),
                  setterForProperty(propertyDescriptor, builderClass),
                  dynamoDbFlatten.prefix());
} else { ... }

flattenedMapper.otherItemTableSchema.attributeNames().forEach(
    attrName -> {
        final String attributeName = flattenedMapper.prefix + attrName
        if (mutableAttributeNames.contains(attributeName)) {
            throw new IllegalArgumentException(
                "Attempt to add an attribute to a mapper that already has one with the same name. " +
                    "[Attribute name: " + attributeName + "]");
        }

        mutableAttributeNames.add(attributeName);
        mutableFlattenedMappers.put(attributeName, flattenedMapper);
    }
);

Other Information

Here is an example test written in Spock:

// Record.java

import java.time.Instant;

import lombok.Data;
import lombok.Getter;
import software.amazon.awssdk.enhanced.dynamodb.mapper.annotations.DynamoDbBean;
import software.amazon.awssdk.enhanced.dynamodb.mapper.annotations.DynamoDbFlatten;
import software.amazon.awssdk.enhanced.dynamodb.mapper.annotations.DynamoDbPartitionKey;

@Data
@DynamoDbBean
public class Record {
    @Getter(onMethod_ = {@DynamoDbPartitionKey})
    String id;
    @Getter(onMethod_ = {@DynamoDbFlatten})
    Edit created;
    @Getter(onMethod_ = {@DynamoDbFlatten})
    Edit updated;

    @Data
    @DynamoDbBean
    public static class Edit {
        String id;
        Instant timestamp;
    }
}

// DynamoDBSpec.groovy

import software.amazon.awssdk.enhanced.dynamodb.DynamoDbEnhancedClient
import software.amazon.awssdk.enhanced.dynamodb.DynamoDbTable
import software.amazon.awssdk.enhanced.dynamodb.TableSchema
import software.amazon.awssdk.enhanced.dynamodb.extensions.AutoGeneratedTimestampRecordExtension
import software.amazon.awssdk.enhanced.dynamodb.extensions.VersionedRecordExtension
import software.amazon.awssdk.services.dynamodb.local.embedded.DynamoDBEmbeddedRule
import software.amazon.awssdk.services.dynamodb.model.AttributeValue
import spock.lang.Shared
import spock.lang.Specification

import java.time.Instant

class DynamoDBSpec extends Specification {
    @Shared
    DynamoDbClient dynamoDbClient = DynamoDbEnhancedClient.create()
    @Shared
    DynamoDbEnhancedClient dynamoDb = DynamoDbEnhancedClient.builder()
            .dynamoDbClient(dynamoDbClient())
            .build()
    @Shared
    DynamoDbTable<Record> recordsTable = dynamoDb.table("Records", TableSchema.fromClass(Record.class))

    void setup() {
        recordsTable.createTable()
    }

    void cleanup() {
        recordsTable.deleteTable()
    }

    def "flattened records should not conflict"() {
        given:
        def created = new Record.Edit().tap {
            id = "Creating User"
            timestamp = Instant.EPOCH
        }
        def updated = new Record.Edit().tap {
            id = "Updating User"
            timestamp = Instant.EPOCH.plusSeconds(10L)
        }
        def record = new Record().tap {
            id = "Record ID"
            it.created = created
            it.updated = updated
        }

        when:
        recordsTable.putItem(record)

        then:
        def item = recordsTable.getItem(record)

        and:
        item == record
    }

}

// Stacktrace

Attempt to add an attribute to a mapper that already has one with the same name. [Attribute name: id]
java.lang.IllegalArgumentException: Attempt to add an attribute to a mapper that already has one with the same name. [Attribute name: id]
	at software.amazon.awssdk.enhanced.dynamodb.mapper.StaticImmutableTableSchema.lambda$null$2(StaticImmutableTableSchema.java:187)
	at java.base/java.util.ArrayList.forEach(ArrayList.java:1541)
	at java.base/java.util.Collections$UnmodifiableCollection.forEach(Collections.java:1085)
	at software.amazon.awssdk.enhanced.dynamodb.mapper.StaticImmutableTableSchema.lambda$new$3(StaticImmutableTableSchema.java:184)
	at java.base/java.util.ArrayList.forEach(ArrayList.java:1541)
	at software.amazon.awssdk.enhanced.dynamodb.mapper.StaticImmutableTableSchema.<init>(StaticImmutableTableSchema.java:182)
	at software.amazon.awssdk.enhanced.dynamodb.mapper.StaticImmutableTableSchema.<init>(StaticImmutableTableSchema.java:80)
	at software.amazon.awssdk.enhanced.dynamodb.mapper.StaticImmutableTableSchema$Builder.build(StaticImmutableTableSchema.java:429)
	at software.amazon.awssdk.enhanced.dynamodb.mapper.StaticTableSchema.<init>(StaticTableSchema.java:69)
	at software.amazon.awssdk.enhanced.dynamodb.mapper.StaticTableSchema.<init>(StaticTableSchema.java:67)
	at software.amazon.awssdk.enhanced.dynamodb.mapper.StaticTableSchema$Builder.build(StaticTableSchema.java:259)
	at software.amazon.awssdk.enhanced.dynamodb.mapper.BeanTableSchema.createStaticTableSchema(BeanTableSchema.java:218)
	at software.amazon.awssdk.enhanced.dynamodb.mapper.BeanTableSchema.create(BeanTableSchema.java:138)
	at software.amazon.awssdk.enhanced.dynamodb.mapper.BeanTableSchema.create(BeanTableSchema.java:129)
	at software.amazon.awssdk.enhanced.dynamodb.TableSchema.fromBean(TableSchema.java:83)
	at software.amazon.awssdk.enhanced.dynamodb.TableSchema.fromClass(TableSchema.java:126)
	at com.amazon.aet.comms.configuration.utils.DynamoDBSpec.setupSpec(DynamoDBSpec.groovy:29)