How to get/fetch certain columns from dynamoDB using python in Lambda?

Question

I have a table called 'DATA' in dynamodb where I have 20 to 25 columns. But I need to pull only 3 columns from dynamodb.
Required columns are status, ticket_id and country

table_name = 'DATA'
# dynamodb client 
dynamodb_client = boto3.client('dynamodb')

Required columns are status, ticket_id I'm able to achieve using scan as provided below. But I want to do the same using query method.

response = table.scan(AttributesToGet=['ticket_id','ticket_status'])

I tried the below code with query method. But I'm getting error.

response = table.query(ProjectionExpression=['ticket_id','ticket_status']),keyConditionExpression('opco_type').eq('cwc') or keyConditionExpression('opco_type').eq('cwp'))

Is there any way of getting only required columns from dynamo?

You need to use a projection expression docs.aws.amazon.com/amazondynamodb/latest/developerguide/… — luk2302
– luk2302, Commented Dec 21, 2021 at 12:06
Yes, I read about it..But got stucked, because there we need to put key value pairs. how can I use it inorder to get all values from specific columns. Please see my updated question. @luk2302 — Mahilan
– Mahilan, Commented Dec 21, 2021 at 12:22

kgiannakakis · Accepted Answer · 2021-12-21 13:35:34Z

2

As already commented, you need to use ProjectExpression:

dynamodb = boto3.resource('dynamodb', region_name=region)
table = dynamodb.Table(table_name)
item = table.get_item(Key={'Title': 'Scarface', 'Year': 1983}, ProjectionExpression='status, ticket_id, country')

Some things to note:

It is better to use resource instead of client. This will avoid special dynamodb json syntax.
You need to set the full (composite) key to get_item
Selected columns should be in a comma-separated string
It is a good idea to always use expression attribute names:

    item = table.get_item(Key={'Title': 'Scarface', 'Year': 1983}, 
                          ProjectionExpression='#status, ticket_id, country',
                          ExpressionAttributeNames={'#status': 'status'})

edited Dec 21, 2021 at 13:35

answered Dec 21, 2021 at 13:25

kgiannakakis

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6 Comments

Mahilan Over a year ago

#response = table.scan(AttributesToGet=['ticket_id','ticket_status']) I should achieve the same using projectionexpression. Any help @kgiannakakis

kgiannakakis Over a year ago

@Mahilan: Make sure that you understand the difference between scan and query.

Mahilan Over a year ago

Please see my updated question. I should fetch only columns with all the values. The above code is working but Is there any way to acheive the same using projectionexpression.

kgiannakakis Over a year ago

You mean get all items that have both ticket_id and ticket_status set? I don't think this is possible without creating a GSI.

Mahilan Over a year ago

yes, correct. Can you pls help me on this

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