"Computed" properties in Ash?

Elixir Framework Forums Ash Forum
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1

Does ash have the concept of “computed” fields? Ie something like

attributes do
  attribute :status, :atom, constraints: [:open, :closed]

  # i know this doesnt exist
  computed :is_open?, :boolean, computation: expr(status == :open)
end

I’m asking because I have a calculation on a resource which is incorrect after updating the resource. Manually refetching all possible calculations seems very error prone and can’t be the recommended way. I saw in the documentation that there is a generated? setting on an attribute but the explanation Whether or not the value may be generated by the data layer. doesnt tell me too much.

My “mutation” looks like this:

ticket =
  Support.Ticket.get_by_id!(ticket_id)
  |> Support.Ticket.toggle!()
  |> Support.load!(:is_open?)

However, this means that I would have to remember to include all the calculations that might possible be needed.

2

:thinking: you mentioned calculations in your final sentence, but I’m not sure if you’re referring to Ash’s calculations or not.

Ash calculations can do what you want, i.e

calculations do
  calculate :is_open?, :boolean, expr(status == :open)
end

Is that not what you’re looking for?

3

Yes kind of. calculations have to be loaded explicitly.

What I’m thinking of is like a getter in a JS class. A property that gets derived on demand and is reactive.

In ash terms it would probably be a computation that is always computed (also when an instance of a resource is returned during a update action).

4

I see. There are some options.

You can add a global preparation and a global change to ensure that a given calculation is always loaded, like so:

preparations do
  prepare build(load: :calc)
end

changes do
  change load(:calc)
end
  1. you can calculate the thing you need on demand:
Api.calculate(record, :is_open?)

And if you want a named function for that, you can put it in your code interface:

code_interface do
  define_for YourApi

  define_calculation :is_open?, args: [:_record]
end

which will give you YourResource.is_open?(your_record).

In cases where it is not necessary, this will not visit the data layer (i.e in your case we have the status, so we compute the result in Elixir). However, loading up front is often good in case calculations may need to do something expensive, you can do them in one place up front.

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