.NET: Document Model

Topics

The AWS SDK for .NET provides document model classes that wrap some of the low-level DynamoDB operations, to further simplify your coding. In the document model, the primary classes are Table and Document. The Table class provides data operation methods such as PutItem, GetItem, and DeleteItem. It also provides the Query and the Scan methods. The Document class represents a single item in a table.

The preceding document model classes are available in the Amazon.DynamoDBv2.DocumentModel namespace.

Operations Not Supported by the Document Model

You cannot use the document model classes to create, update, and delete tables. The document model does support most common data operations, however.

Getting an Item - Table.GetItem

The GetItem operation retrieves an item as a Document instance. You must provide the primary key of the item that you want to retrieve as shown in the following C# code snippet:

Example



Table table = Table.LoadTable(client, "ProductCatalog");
Document document = table.GetItem(101); // Primary key 101.

The GetItem operation returns all the attributes of the item and performs an eventually consistent read (see Read Consistency) by default.

Specifying Optional Parameters

You can configure additional options for the GetItem operation by adding the GetItemOperationConfig parameter. For a complete list of optional parameters, see GetItem. The following C# code snippet retrieves an item from the ProductCatalog table. It specifies the GetItemOperationConfig to provide the following optional parameters:

The AttributesToGet parameter to retrieve only the specified attributes.
The ConsistentRead parameter to request the latest values for all the specified attributes. To learn more about data consistency, see Read Consistency.

Example



Table table = Table.LoadTable(client, "ProductCatalog");

GetItemOperationConfig config = new GetItemOperationConfig()
{
  AttributesToGet = new List<string>() { "Id", "Title", "Authors", "InStock", "QuantityOnHand" },
  ConsistentRead = true
};
Document doc = table.GetItem(101, config);

When you retrieve an item using the document model API, you can access individual elements within the Document object is returned:

Example



int id = doc["Id"].AsInt();
string title = doc["Title"].AsString();
List<string> authors = doc["Authors"].AsListOfString();
bool inStock = doc["InStock"].AsBoolean();
DynamoDBNull quantityOnHand = doc["QuantityOnHand"].AsDynamoDBNull();

For attributes that are of type List or Map, here is how to map these attributes to the document model API:

List — use the AsDynamoDBList method.
Map — use the AsDocument method.

The following code snippet shows how to retrieve a List (RelatedItems) and a Map (Pictures) from the Document object:

Example



DynamoDBList relatedItems = doc["RelatedItems"].AsDynamoDBList();
            
Document pictures = doc["Pictures"].AsDocument();

Deleting an Item - Table.DeleteItem

The DeleteItem operation deletes an item from a table. You can either pass the item's primary key as a parameter or if you have already read an item and have the corresponding Document object, you can pass it as a parameter to the DeleteItem method as shown in the following C# code snippet.

Example



Table table = Table.LoadTable(client, "ProductCatalog");

// Retrieve a book (a Document instance)
Document document = table.GetItem(111); 

// 1) Delete using the Document instance.
 table.DeleteItem(document);

// 2) Delete using the primary key.
int partitionKey = 222;
table.DeleteItem(partitionKey)

Specifying Optional Parameters

You can configure additional options for the Delete operation by adding the DeleteItemOperationConfig parameter. For a complete list of optional parameters, see DeleteTable. The following C# code snippet specifies the two following optional parameters:

The ConditionalExpression parameter to ensure that the book item being deleted has a specific value for the ISBN attribute.
The ReturnValues parameter to request that the Delete method return the item that it deleted.

Example



Table table = Table.LoadTable(client, "ProductCatalog");
int partitionKey = 111;

Expression expr = new Expression();
expr.ExpressionStatement = "ISBN = :val";
expr.ExpressionAttributeValues[":val"] = "11-11-11-11";

// Specify optional parameters for Delete operation.
DeleteItemOperationConfig config = new DeleteItemOperationConfig
{
     ConditionalExpression = expr,
     ReturnValues = ReturnValues.AllOldAttributes // This is the only supported value when using the document model.
};

// Delete the book.
Document d = table.DeleteItem(partitionKey, config);

Updating an Item - Table.UpdateItem

The UpdateItem operation updates an existing item if it is present. If the item that has the specified primary key is not found, the UpdateItem operation adds a new item.

You can use the UpdateItem operation to update existing attribute values, add new attributes to the existing collection, or delete attributes from the existing collection. You provide these updates by creating a Document instance that describes the updates you wish to perform.

The UpdateItem action uses the following guidelines:

If the item does not exist, UpdateItem adds a new item using the primary key that is specified in the input.
If the item exists, UpdateItem applies the updates as follows:
- Replaces the existing attribute values with the values in the update.
- If an attribute that you provide in the input does not exist, it adds a new attribute to the item.
- If the input attribute value is null, it deletes the attributes, if it is present.

Note

This mid-level UpdateItem operation does not support the Add action (see UpdateItem) supported by the underlying DynamoDB operation.

Note

The PutItem operation (Putting an Item - Table.PutItem Method) can also perform an update. If you call PutItem to upload an item and the primary key exists, the PutItem operation replaces the entire item. Note that, if there are attributes in the existing item and those attributes are not specified on the Document that is being put, the PutItem operation deletes those attributes. However, UpdateItem only updates the specified input attributes. Any other existing attributes of that item will remain unchanged.

The following are the steps to update an item using the AWS SDK for .NET document model.

Execute the Table.LoadTable method by providing the name of the table in which you want to perform the update operation.
Create a Document instance by providing all the updates that you wish to perform.

To delete an existing attribute, specify the attribute value as null.
Call the Table.UpdateItem method and provide the Document instance as an input parameter.

You must provide the primary key either in the Document instance or explicitly as a parameter.

The following C# code snippet demonstrates the preceding tasks. The code sample updates an item in the Book table. The UpdateItem operation updates the existing Authors attribute, deletes the PageCount attribute, and adds a new attribute XYZ. The Document instance includes the primary key of the book to update.

Example



Table table = Table.LoadTable(client, "ProductCatalog");

var book = new Document();

// Set the attributes that you wish to update.
book["Id"] = 111; // Primary key.
// Replace the authors attribute.
book["Authors"] = new List<string> { "Author x", "Author y" }; 
// Add a new attribute.   
book["XYZ"] = 12345; 
// Delete the existing PageCount attribute.
book["PageCount"] = null;

table.Update(book);

Specifying Optional Parameters

You can configure additional options for the UpdateItem operation by adding the UpdateItemOperationConfig parameter. For a complete list of optional parameters, see UpdateItem.

The following C# code snippet updates a book item price to 25. It specifies the two following optional parameters:

The ConditionalExpression parameter that identifies the Price attribute with value 20 that you expect to be present.
The ReturnValues parameter to request the UpdateItem operation to return the item that is updated.

Example



Table table = Table.LoadTable(client, "ProductCatalog");
string partitionKey = "111";

var book = new Document();
book["Id"] = partitionKey;
book["Price"] = 25;

Expression expr = new Expression();
expr.ExpressionStatement = "Price = :val";
expr.ExpressionAttributeValues[":val"] = "20";

UpdateOperationConfig config = new UpdateOperationConfig()
{
    ConditionalExpression = expr,
    ReturnValues = ReturnValues.AllOldAttributes
};

Document d1 = table.Update(book, config);

Batch Write - Putting and Deleting Multiple Items

Batch write refers to putting and deleting multiple items in a batch. The operation enables you to put and delete multiple items from one or more tables in a single call. The following are the steps to put or delete multiple items from a table using the AWS SDK for .NET document model API.

Create a Table object by executing the Table.LoadTable method by providing the name of the table in which you want to perform the batch operation.
Execute the CreateBatchWrite method on the table instance you created in the preceding step and create DocumentBatchWrite object.
Use DocumentBatchWrite object methods to specify documents you wish to upload or delete.
Call the DocumentBatchWrite.Execute method to execute the batch operation.

When using the document model API, you can specify any number of operations in a batch. However, note that DynamoDB limits the number of operations in a batch and the total size of the batch in a batch operation. For more information about the specific limits, see BatchWriteItem. If the document model API detects your batch write request exceeded the number of allowed write requests or the HTTP payload size of a batch exceeded the limit allowed by BatchWriteItem, it breaks the batch in to several smaller batches. Additionally, if a response to a batch write returns unprocessed items, the document model API will automatically send another batch request with those unprocessed items.

The following C# code snippet demonstrates the preceding steps. The code snippet uses batch write operation to perform two writes; upload a book item and delete another book item.



Table productCatalog = Table.LoadTable(client, "ProductCatalog");
var batchWrite = productCatalog.CreateBatchWrite();

var book1 = new Document();
book1["Id"] = 902;
book1["Title"] = "My book1 in batch write using .NET document model";
book1["Price"] = 10;
book1["Authors"] = new List<string> { "Author 1", "Author 2", "Author 3" };
book1["InStock"] = new DynamoDBBool(true);
book1["QuantityOnHand"] = 5;

batchWrite.AddDocumentToPut(book1);
// specify delete item using overload that takes PK. 
batchWrite.AddKeyToDelete(12345); 

batchWrite.Execute();

For a working example, see Example: Batch Operations Using AWS SDK for .NET Document Model API.

You can use the batch write operation to perform put and delete operations on multiple tables. The following are the steps to put or delete multiple items from multiple table using the AWS SDK for .NET document model.

You create DocumentBatchWrite instance for each table in which you want to put or delete multiple items as described in the preceding procedure.
Create an instance of the MultiTableDocumentBatchWrite and add the individual DocumentBatchWrite objects in it.
Execute the MultiTableDocumentBatchWrite.Execute method.

The following C# code snippet demonstrates the preceding steps. The code snippet uses batch write operation to perform the following write operations:

Put a new item in the Forum table item
Put an item in the Thread table and delete an item from the same table.



// 1. Specify item to add in the Forum table.
Table forum = Table.LoadTable(client, "Forum");
var forumBatchWrite = forum.CreateBatchWrite();

var forum1 = new Document();
forum1["Name"] = "Test BatchWrite Forum";
forum1["Threads"] = 0;
forumBatchWrite.AddDocumentToPut(forum1); 


// 2a. Specify item to add in the Thread table.
Table thread = Table.LoadTable(client, "Thread");
var threadBatchWrite = thread.CreateBatchWrite();
  
var thread1 = new Document();
thread1["ForumName"] = "Amazon S3 forum";
thread1["Subject"] = "My sample question";
thread1["Message"] = "Message text";
thread1["KeywordTags"] = new List<string>{ "Amazon S3", "Bucket" };
threadBatchWrite.AddDocumentToPut(thread1);

// 2b. Specify item to delete from the Thread table.
threadBatchWrite.AddKeyToDelete("someForumName", "someSubject");

// 3. Create multi-table batch.
var superBatch = new MultiTableDocumentBatchWrite();
superBatch.AddBatch(forumBatchWrite);
superBatch.AddBatch(threadBatchWrite);

superBatch.Execute();

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