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Expression attribute names in DynamoDB

An expression attribute name is a placeholder that you use in an Amazon DynamoDB expression as an alternative to an actual attribute name. An expression attribute name must begin with a pound sign (#), and be followed by one or more alphanumeric characters.

This section describes several situations in which you must use expression attribute names.

Reserved words

Sometimes you might need to write an expression containing an attribute name that conflicts with a DynamoDB reserved word. (For a complete list of reserved words, see Reserved words in DynamoDB.)

For example, the following Amazon CLI example would fail because COMMENT is a reserved word.

aws dynamodb get-item \
    --table-name ProductCatalog \
    --key '{"Id":{"N":"123"}}' \
    --projection-expression "Comment"

To work around this, you can replace Comment with an expression attribute name such as #c. The # (pound sign) is required and indicates that this is a placeholder for an attribute name. The Amazon CLI example would now look like the following.

aws dynamodb get-item \
     --table-name ProductCatalog \
     --key '{"Id":{"N":"123"}}' \
     --projection-expression "#c" \
     --expression-attribute-names '{"#c":"Comment"}'

Attribute names containing special characters

In an expression, a dot (".") is interpreted as a separator character in a document path. However, DynamoDB also allows you to use a dot character and other special characters, such as a hyphen ("-") as part of an attribute name. This can be ambiguous in some cases. To illustrate, suppose that you wanted to retrieve the Safety.Warning attribute from a ProductCatalog item (see Specifying item attributes when using expressions).

Suppose that you wanted to access Safety.Warning using a projection expression.

aws dynamodb get-item \
    --table-name ProductCatalog \
    --key '{"Id":{"N":"123"}}' \
    --projection-expression "Safety.Warning"

DynamoDB would return an empty result, rather than the expected string ("Always wear a helmet"). This is because DynamoDB interprets a dot in an expression as a document path separator. In this case, you must define an expression attribute name (such as #sw) as a substitute for Safety.Warning. You could then use the following projection expression.

aws dynamodb get-item \
    --table-name ProductCatalog \
    --key '{"Id":{"N":"123"}}' \
    --projection-expression "#sw" \
    --expression-attribute-names '{"#sw":"Safety.Warning"}'

DynamoDB would then return the correct result.

Nested attributes

Suppose that you wanted to access the nested attribute ProductReviews.OneStar, using the following projection expression.

aws dynamodb get-item \
    --table-name ProductCatalog \
    --key '{"Id":{"N":"123"}}' \
    --projection-expression "ProductReviews.OneStar"

The result would contain all of the one-star product reviews, which is expected.

But what if you decided to use an expression attribute name instead? For example, what would happen if you were to define #pr1star as a substitute for ProductReviews.OneStar?

aws dynamodb get-item \
    --table-name ProductCatalog \
    --key '{"Id":{"N":"123"}}' \
    --projection-expression "#pr1star"  \
    --expression-attribute-names '{"#pr1star":"ProductReviews.OneStar"}'

DynamoDB would return an empty result instead of the expected map of one-star reviews. This is because DynamoDB interprets a dot in an expression attribute name as a character within an attribute's name. When DynamoDB evaluates the expression attribute name #pr1star, it determines that ProductReviews.OneStar refers to a scalar attribute—which is not what was intended.

The correct approach would be to define an expression attribute name for each element in the document path:

You could then use #pr.#1star for the projection expression.

aws dynamodb get-item \
    --table-name ProductCatalog \
    --key '{"Id":{"N":"123"}}' \
    --projection-expression "#pr.#1star"  \
    --expression-attribute-names '{"#pr":"ProductReviews", "#1star":"OneStar"}'

DynamoDB would then return the correct result.

Repeating attribute names

Expression attribute names are helpful when you need to refer to the same attribute name repeatedly. For example, consider the following expression for retrieving some of the reviews from a ProductCatalog item.

aws dynamodb get-item \
    --table-name ProductCatalog \
    --key '{"Id":{"N":"123"}}' \
    --projection-expression "ProductReviews.FiveStar, ProductReviews.ThreeStar, ProductReviews.OneStar"

To make this more concise, you can replace ProductReviews with an expression attribute name such as #pr. The revised expression would now look like the following.

aws dynamodb get-item \
    --table-name ProductCatalog \
    --key '{"Id":{"N":"123"}}' \
    --projection-expression "#pr.FiveStar, #pr.ThreeStar, #pr.OneStar" \
    --expression-attribute-names '{"#pr":"ProductReviews"}'

If you define an expression attribute name, you must use it consistently throughout the entire expression. Also, you cannot omit the # symbol.