Using default on non-saved attributes when parsing query results

[Jimmy89]

Hello all,

I have the following issue:
I have an entity called CreatedAt, storing for specific types a CreatedAt record (for a specific sortkey lookup). One of my attributes is named createdAt, which is the timestamp stored within the ULID (stored within the id field).

I thought I could be smart (and saving a few bits of dynamodb space) by not storing the createdAt attribute, but to calculate it based on the id like this:

// Entity details
attributes: {
   // Other attributes, including `id`
   createdAt: { type: "number", map: "CA", dependsOn: ["id"], transform: (_, {id}) => epochIntoISO(parseId(id).epoch, true), default: ({id}) => epochIntoISO(parseId(id).epoch, true), save: false }, // For clarity, first I mapped it to a CA attribute, but then I added `save: false`.
}

I was expecting that when I use the CreatedAt.query( /* query details */ ) the parsed record would contain a createdAt attribute as well. Unfortunately, it does not. I was hoping it would fire the default, but I guess because there is no CA attribute, it does not trigger this. Is there a way to resolve this issue?

[naorpeled]

Hey @Jimmy89,
Afaik, we don't support virtual fields.
Meaning you'd need to store it in DDB,
default is used in put and update operations,
transform is called only when fields are not undefined.

Definitely something we should be adding.
Will discuss this with Thomas and Jeremy and see if they think it's worth adding this to this version or only v1.

[rakesh9541]

can you assign this to me and tell me the file of it

[naorpeled]

can you assign this to me and tell me the file of it

Hey @rakesh9541,
thanks for volunteering.
Sure, the thing is that I don't have a solid strategy for this yet 😢

If you want to work on something else in the meantime,
this pr is up for grabs.

[rakesh9541]

@naorpeled ok i will have a look