DynamoDB：如果属性已存在，如何更新除一个属性之外的项目

Question

So I'm trying to put an Item into a DDB table (nodejs). If the Item has a 'date_created' attribute, I want to update all fields in that Item except for the 'date_created'.

I've looked at conditional expressions and from what I understand it's pretty binary. If condition == true then proceed, if not, then don't. What I'm looking for is to do the put no matter what but don't update 'date_created' if it exists.

Is this possible? Am I even approaching this with the right mindset?

Answer 1

You don't want a PutItem with a ConditionExpression for this case.

You need to use the UpdateItem API. UpdateItem is an "upset", so if the primary key does not exist, the API will cause a new item to be inserted. This API accepts an Update Expression which can be used to set, modify, and/or remove one or more fields from the item. Update Expressions have a few special functions available to them, and in order to prevent overwriting existing data, you should use if_not_exists(path, operand) . If path does not exist, DynamoDB will put the result of operand at path . If path does exist, it will be unchanged.

The update expression for your case would look something like this:

SET if_not_exists(date_created, :right_now), other_field_1 = :value_1, other_field_2 = :value_2

Answer 2

I don't believe DynamoDB supports this natively.

A better solution might have date_created on initial put, and date_updated on subsequent updates. The initial put is a consequence of some high-level 'new item' kind of operation (like 'Add Employee'). The subsequent updates don't need to be conditional -- they don't include date_created , just date_updated . And you hide those two attributes within your model layer (that implements createRecord() and updateRecord() , for example).

Alternatively, do an initial DDB put, including date_created , that is conditional on date_created being absent. If this conditional request fails, do an update without date_created .

Answer 3

I agree with @jarmod on the best way to do this; however, if you really want to do an update on all fields except the one field you can do an update instead of a put . In JavaScript you could even generate the update statement based on the keys in the data, and exclude the date_created value if it happens to be in the data.