Derived state

[codingnuclei]

Hi,

Is it possible to have an entity with derived state?

i.e. if an item in a table has the attributes: oneStar twoStar threeStar fourStar fiveStar. When i do a entity.get() (once has fetch the item) a new attribute rating is calculated based off the numbers stored in the attributes previously mentioned. This derived state should not be saved/exist in the table.

I have played around with default and format etc but have had no joy

[naorpeled]

Hey @codingnuclei, first of all, amazing initiative.

This is something that we've discussed in the past but haven't reached a final verdict on.

@jeremydaly @ThomasAribart wdyt?

[codingnuclei]

Hey - so I have added quite a few things now to: https://github.com/jeremydaly/dynamodb-toolbox/pull/689

I think it gives a real flavour of what I am proposing. It is pretty non-intrusive with patterns and existing code.

Would really value some feedback as to whether this is something the team wants within the toolbox.

😄

[jeremydaly]

This should be possible with format. It receives both the value of the attribute AND the entire object. You should be able to conditionally return based off of other attributes.

[codingnuclei]

Hey - yea i was initially hoping to use the format functionality but wasn't able to get it working.

From what I can see, the formatItem function loops over the response from the DB and matches the response fields against the entity schema:

https://github.com/jeremydaly/dynamodb-toolbox/blob/e527a1af512093b1d44add9944c8c5011e5f6feb/src/lib/formatItem.ts#L63

https://github.com/jeremydaly/dynamodb-toolbox/blob/e527a1af512093b1d44add9944c8c5011e5f6feb/src/lib/formatItem.ts#L108-L110

So my understanding is that if the field is not in the response Item then what is declared in the entity makes no difference...

Hoping I am missing something :)

[codingnuclei]

@jeremydaly any thoughts on the above? 😀

[ThomasAribart]

@codingnuclei I'm thinking about adding readDefaults and readLinks at some point in the future as it is highly requested:

const schema = item({
  oneStar: number(),
  twoStar: number(),
  threeStar: number(),
  fourStar: number(),
  fiveStar: number(),
}).and(prevSchema => ({
  rating: number().optional().readLink<typeof prevSchema>(stars => getRating(stars))
})

Although I just realized that will probably require a breaking change on the required syntax first to add this edge case:

// prev
const attribute = string().required('always')

// next
const attribute = string().optional().readRequired() // or .putRequired(), or .updateRequired()