DynamoDB update item with conditionExpression instead of key - node.js

Question

I try to update an item in dynamodb by adding a condition, without passing the key in the parameters.
And as soon as my condition is true update. Is it possible to do this?
Below an example of an item:
{
"id" : "bcc2f32e-305e-4469-88e2-463724b5c6a9",
"name" : "toto",
"email" : "toto#titi.com"
}
Where email is unique for items.
I tested this code and it works :
const name= "updateName";
const params = {
TableName: MY_TABLE,
Key: {
id
},
UpdateExpression: 'set #name = :name',
ExpressionAttributeNames: { '#name': 'name' },
ExpressionAttributeValues: { ':name': name },
ReturnValues: "ALL_NEW"
}
dynamoDb.update(params, (error, result) => {
if (error) {
res.status(400).json({ error: 'Could not update Item' });
}
res.json(result.Attributes);
})
But i want to do something like this (replace the Key by conditionExpression):
const params = {
TableName: MY_TABLE,
UpdateExpression: 'set #name = :name',
ConditionExpression: '#email = :email',
ExpressionAttributeNames: {
'#name': 'name',
'#email': 'email'
},
ExpressionAttributeValues: {
':name': name,
':email': email
},
ReturnValues: "ALL_NEW"
}
dynamoDb.update(params, (error, result) => {
if (error) {
res.status(400).json({ error: 'Could not update User' });
}
res.json(result.Attributes);
})
But this code doesn't work.
Any ideas?

Answer 1

You cannot update an item in DynamoDB without using the entire primary key (partition key, and sort key if present). This is because you must specify exactly one record for the update. See the documentation here.
If you want to find an item using a field that is not the primary key, then you can search using a scan (potentially slow and expensive) or by using a Global Secondary Index (GSI) on that field. Either of these methods requires that you do a separate request to find the item in question, and then use its primary key to perform the update.
It sounds like you want to do an update that waits for a condition. That's not how DynamoDb works; it cannot wait for anything (except consistency, I suppose, but that's somewhat different). What you can do is make a request with a condition, and if it fails the condition (returning immediately), make the request again later. If you do this you'll need to be careful to backoff appropriately, or you might end up making a lot of requests very quickly.

Answer 2

The key is a required parameter when doing updates; the condition expression can be used in addition to providing the key, but can't be used instead of the key.
Also, I am not sure you fully understand what the conditionExpression is for - its not like the 'where' clause in an SQL update statement (i.e. update mytable set name='test' where email='myemail.com'.
Instead, logically the conditionExpression in an update would be more like:
update mytable set name='test' where key='12345' but only if quantity >0 - for example,
i.e. you are telling dynamodb the exact key of the record you want updated, and once it finds it it uses the condition expression to determine if the update should proceed - i.e. find the record with id=12345, and change the name to 'test', only of the quantity is greater than 0.
It does not use the conditionExpression to find records to update.