DynamoDB supports an UpdateItem
operation that modifies an existing or creates a new item. The UpdateItem
accepts an UpdateExpression
that dictates which operations will occur on the specified item.
In this post, we'll explore how to perform multiple operations including within a clause/keyword and with multiple clauses/keywords.
What is an update expression
An update expression supports four keywords:
-
SET
(modify or add item attributes) -
REMOVE
(deleting attributes from an item) -
ADD
(updating numbers and sets) -
DELETE
(removing elements from a set)
The syntax for an update expression is as follows:
update-expression ::=
[ SET action [, action] ... ]
[ REMOVE action [, action] ...]
[ ADD action [, action] ... ]
[ DELETE action [, action] ...]
As you can view, DynamoDB supports four main clauses that begin with one of the specified operations.
For a detailed explanation of update expressions, please refer to the AWS documentation.
For demonstration purposes, I'm going to create a DynamoDB table named books
with a number hash key named id
.
aws dynamodb create-table \
--table-name books \
--attribute-definitions AttributeName=id,AttributeType=N \
--key-schema AttributeName=id,KeyType=HASH \
--provisioned-throughput ReadCapacityUnits=5,WriteCapacityUnits=5
I'll also add an item to the table:
{
"id": {
"N": "1"
},
"title": {
"S": "Iliad"
},
"author": {
"S": "Homer"
},
"genre": {
"S": "Epic Poetry"
},
"publication_year": {
"N": "1488"
}
}
aws dynamodb put-item \
--table-name books \
--item file://book.json
How to specify a single keyword in DynamoDB update expression
Let's showcase a basic use-case of adding an element to an existing item:
aws dynamodb update-item \
--table-name books \
--key '{"id":{"N":"1"}}' \
--update-expression "SET category = :c" \
--expression-attribute-values '{":c":{"S":"Poem"}}'
Now the item has a new category
attribute:
aws dynamodb get-item \
--table-name books \
--key '{"id":{"N":"1"}}' \
--projection-expression "category"
Output:
{
"Item": {
"category": {
"S": "Poem"
}
}
}
How to specify a single keyword with multiple actions in DynamoDB update expression
To showcase mutliple SET
operations in a single DynamoDB update expression, we'll use a comma (,
) to separate them.
In the following example, we're changing the category
from poem
-> poetry
and adding a new element named genre
.
aws dynamodb update-item \
--table-name books \
--key '{"id":{"N":"1"}}' \
--update-expression "SET category = :c, genre = :g" \
--expression-attribute-values '{":c":{"S":"poetry"}, ":g":{"S":"Epic poetry"}}' \
--return-values UPDATED_NEW
Output:
{
"Attributes": {
"category": {
"S": "poetry"
},
"genre": {
"S": "Epic poetry"
}
}
}
As you see, the update-expression
includes a comma between the two operations:
SET category = :c, genre = :g
How to specify multiple keywords with multiple actios in DynamoDB update expression
Lastly, let's showcase leveraging multiple keywords with their respective actions.
We're going to perform the following:
- Set a new string attribute named
product_category
with a value ofpoem
- Remove the
category
attribute - Add
1
to existingcheckout_total
number or initialize it with the number value of1
- Add
Trojan War
to existingsubjects
set or initialize it with the first index being string value ofTrojan War
aws dynamodb update-item \
--table-name books \
--key '{"id":{"N":"1"}}' \
--update-expression "SET #pc = :pc REMOVE #c ADD #ct :inc, #s :s" \
--expression-attribute-names '{"#s":"subjects","#pc":"product_category","#c":"category","#ct":"checkout_total"}' \
--expression-attribute-values '{":s":{"SS":["Trojan War"]},":pc":{"S":"poem"},":inc":{"N":"1"}}' \
--return-values UPDATED_NEW
Output:
{
"Attributes": {
"checkout_total": {
"N": "1"
},
"subjects": {
"SS": [
"Trojan War"
]
},
"product_category": {
"S": "poem"
}
}
}
It's important to note that because the ADD
operation will continue incrementing and the SET
data structure doesn't allow duplicates, we can rerun the same operation and checkout_total
will increase by 1
.
For readability, let's substitute the expression-attribute-names
and expression-attribute-values
back into the update-expression
:
SET product_category = poem REMOVE category ADD checkout_total 1, subjects ["Trojan War"]
To summarize, using mutliple clauses in a DynamoDB UpdateExpression
is supported by simply separating the clauses by keywords.
If you want to perform multiple operations using the same clause, use a comma (,
) to separate them.
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