DynamoDB - Dynamically Build an Update Expression

update-example.js

const generateUpdateQuery = (fields) => {
    let exp = {
        UpdateExpression: 'set',
        ExpressionAttributeNames: {},
        ExpressionAttributeValues: {}
    }
    Object.entries(fields).forEach(([key, item]) => {
        exp.UpdateExpression += ` #${key} = :${key},`;
        exp.ExpressionAttributeNames[`#${key}`] = key;
        exp.ExpressionAttributeValues[`:${key}`] = item
    })
    exp.UpdateExpression = exp.UpdateExpression.slice(0, -1); 
    return exp
}

let data = {
    'field' : { 'subfield': 123 },
    'other': '456'
}

let expression = generateUpdateQuery(data)

let params = {
    // Key, Table, etc..
    ...expression
}

console.log(params)

Outputs:

{ UpdateExpression: 'set #field = :field, #other = :other,',
  ExpressionAttributeNames: { '#field': 'field', '#other': 'other' },
  ExpressionAttributeValues: { ':field': { subfield: 123 }, ':other': '456' } }

Nice! Super useful appreciate you sharing

For single field updates, the trim function doesn't work on line 12. I would recommend changing it to a regex expression like so:

exp.UpdateExpression = exp.UpdateExpression.replace(/(^,)|(,$)/g, "")

For single field updates, the trim function doesn't work on line 12. I would recommend changing it to a regex expression like so:

exp.UpdateExpression = exp.UpdateExpression.replace(/(^,)|(,$)/g, "")

Hi Mason, thanks for the feedback, however I can't spot the issue you're describing. Can you give an example of a "single field update"? Thanks.

Edit: NM, I see the issue! trim only removes whitespace. I'm not sure where I got the idea that it accepted an argument...

it works. btw, does it support nested objects?

it works. btw, does it support nested objects?

It does not. If you're looking to update nested objects on a regular basis, then I suggest you consider using a different DB.

Edit: To be clear, what I mean is, if you want to update specific nested fields. If you're happy to update entire objects this works fine.

Thanks @daverickdunn! Exactly what I was looking for!

Thanks @daverickdunn .. it saves me a lot

This was useful. Thank you very much.
Added a python implementation of the same logic

def generate_update_query(fields):
    exp = {
        "UpdateExpression": 'set',
        "ExpressionAttributeNames": {},
        "ExpressionAttributeValues": {}
    }
    for key, value in fields.items():
        exp["UpdateExpression"] += f" #{key} = :{key},"
        exp["ExpressionAttributeNames"][f"#{key}"] = key
        exp["ExpressionAttributeValues"][f":{key}"] = value
    exp["UpdateExpression"] = exp["UpdateExpression"][0:-1] 
    return exp