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: #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:

dynamodb-toolbox/src/lib/formatItem.ts

Line 63 in e527a1a

return Object.keys(item).reduce((acc, field) => {

dynamodb-toolbox/src/lib/formatItem.ts

Lines 108 to 110 in e527a1a

	const transformedValue =
	attributes[field] && attributes[field].format
	? (attributes[field] as Required<PureAttributeDefinition>).format(fieldValue, item)

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? 😀