cloudkeys-go/vendor/github.com/aws/aws-sdk-go/example/service/dynamodb/expression

Example

scan is an example how to use Amazon DynamoDB's expression package to fill the member fields of Amazon DynamoDB's Operation input types.

Representing DynamoDB Expressions

In the example, the variable filt represents a FilterExpression. Note that DynamoDB item attributes are represented using the function Name() and DynamoDB item values are similarly represented using the function Value(). In this context, the string "Artist" represents the name of the item attribute that we want to evaluate and the string "No One You Know" represents the value we want to evaluate the item attribute against. The relationship between the two operands are specified using the method Equal().

Similarly, the variable proj represents a ProjectionExpression. The list of item attribute names comprising the ProjectionExpression are specified as arguments to the function NamesList(). The expression package utilizes the type safety of Go and if an item value were to be used as an argument to the function NamesList(), a compile time error is returned. The pattern of representing DynamoDB Expressions by indicating relationships between operands with functions is consistent throughout the whole expression package.

filt := expression.Name("Artist").Equal(expression.Value("No One You Know"))
// let :a be an ExpressionAttributeValue representing the string "No One You Know"
// equivalent FilterExpression: "Artist = :a"

proj := expression.NamesList(expression.Name("SongTitle"), expression.Name("AlbumTitle"))
// equivalent ProjectionExpression: "SongTitle, AlbumTitle"

Creating an Expression

In the example, the variable expr is an instance of an Expression type. An Expression is built using a builder pattern. First, a new Builder is initialized by the NewBuilder() function. Then, types representing DynamoDB Expressions are added to the Builder by methods WithFilter() and WithProjection(). The Build() method returns an instance of an Expression and an error. The error will be either an InvalidParameterError or an UnsetParameterError.

filt := expression.Name("Artist").Equal(expression.Value("No One You Know"))
proj := expression.NamesList(expression.Name("SongTitle"), expression.Name("AlbumTitle"))

expr, err := expression.NewBuilder().WithFilter(filt).WithProjection(proj).Build()
if err != nil {
  fmt.Println(err)
}

Filling in the fields of a DynamoDB Scan API

In the example, the getter methods of the Expression type is used to get the formatted DynamoDB Expression strings. The ExpressionAttributeNames and ExpressionAttributeValues member field of the DynamoDB API must always be assigned when using an Expression since all item attribute names and values are aliased. That means that if the ExpressionAttributeNames and ExpressionAttributeValues member is not assigned with the corresponding Names() and Values() methods, the DynamoDB operation will run into a logic error.

filt := expression.Name("Artist").Equal(expression.Value("No One You Know"))
proj := expression.NamesList(expression.Name("SongTitle"), expression.Name("AlbumTitle"))
expr, err := expression.NewBuilder().WithFilter(filt).WithProjection(proj).Build()
if err != nil {
  fmt.Println(err)
}

input := &dynamodb.ScanInput{
  ExpressionAttributeNames:  expr.Names(),
  ExpressionAttributeValues: expr.Values(),
  FilterExpression:          expr.Filter(),
  ProjectionExpression:      expr.Projection(),
  TableName:                 aws.String("Music"),
}

Usage

go run -tags example scan.go -table "<table_name>" -region "<optional_region>"

Output

{
	Count: #SomeNumber,
	Items: [{
		AlbumTitle: {
			#SomeAlbumTitle
		},
		SongTitle: {
			#SomeSongTitle
		}
	}],
	...
	ScannedCount: #SomeNumber,
}