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Working with the .NET object persistence model and DynamoDB

The AWS SDK for .NET provides an object persistence model that enables you to map your client-side classes to Amazon DynamoDB tables. Each object instance then maps to an item in the corresponding tables. To save your client-side objects to the tables, the object persistence model provides the DynamoDBContext class, an entry point to DynamoDB. This class provides you a connection to DynamoDB and enables you to access tables, perform various CRUD operations, and run queries.

The object persistence model provides a set of attributes to map client-side classes to tables, and properties/fields to table attributes.

The object persistence model does not provide an API to create, update, or delete tables. It provides only data operations. You can use only the AWS SDK for .NET low-level API to create, update, and delete tables.

The following example shows how the object persistence model works. It starts with the ProductCatalog table. It has Id as the primary key.

ProductCatalog(Id, ...)

Suppose that you have a Book class with Title, ISBN, and Authors properties. You can map the Book class to the ProductCatalog table by adding the attributes defined by the object persistence model, as shown in the following C# code example.

[DynamoDBTable("ProductCatalog")]
  public class Book
  {
    [DynamoDBHashKey]
    public int Id { get; set; }

    public string Title { get; set; }
    public int ISBN { get; set; }

    [DynamoDBProperty("Authors")]
    public List<string> BookAuthors { get; set; }

    [DynamoDBIgnore]
    public string CoverPage { get; set; }
  }

In the preceding example, the DynamoDBTable attribute maps the Book class to the ProductCatalog table.

The object persistence model supports both the explicit and default mapping between class properties and table attributes.

• Explicit mapping—To map a property to a primary key, you must use the DynamoDBHashKey and DynamoDBRangeKey object persistence model attributes. Additionally, for the nonprimary key attributes, if a property name in your class and the corresponding table attribute to which you want to map it are not the same, you must define the mapping by explicitly adding the DynamoDBProperty attribute.

  In the preceding example, the Id property maps to the primary key with the same name, and the BookAuthors property maps to the Authors attribute in the ProductCatalog table.

• Default mapping—By default, the object persistence model maps the class properties to the attributes with the same name in the table.

  In the preceding example, the properties Title and ISBN map to the attributes with the same name in the ProductCatalog table.

You don't have to map every single class property. You identify these properties by adding the DynamoDBIgnore attribute. When you save a Book instance to the table, the DynamoDBContext does not include the CoverPage property. It also does not return this property when you retrieve the book instance.

You can map properties of .NET primitive types such as int and string. You also can map any arbitrary data types as long as you provide an appropriate converter to map the arbitrary data to one of the DynamoDB types. To learn about mapping arbitrary types, see Mapping arbitrary data with DynamoDB using the AWS SDK for .NET object persistence model.

The object persistence model supports optimistic locking. During an update operation, this ensures that you have the latest copy of the item you are about to update. For more information, see Optimistic locking using DynamoDB and the AWS SDK for .NET object persistence model.

For more information, see the topics below.

The object persistence model supports a set of primitive .NET data types, collections, and arbitrary data types. The model supports the following primitive data types.

• bool

• byte

• char

• DateTime

• decimal

• double

• float

• Int16

• Int32

• Int64

• SByte

• string

• UInt16

• UInt32

• UInt64

The object persistence model also supports the .NET collection types. DynamoDBContext is able to convert concrete collection types and simple Plain Old CLR Objects (POCOs).

The following table summarizes the mapping of the preceding .NET types to the DynamoDB types.

All number types

N (number type)

All string types

S (string type)

MemoryStream, byte[]

B (binary type)

The object persistence model also supports arbitrary data types. However, you must provide converter code to map the complex types to the DynamoDB types.

• Empty binary values are supported.

• Reading of empty string values is supported. Empty string attribute values are supported within attribute values of string Set type while writing to DynamoDB. Empty string attribute values of string type and empty string values contained within List or Map type are dropped from write requests

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