How do I use Ash.Transform

Questions & Help Questions
1

I need to shift the time_zone of some (DateTime) attributes from a resource. Ideally, I’d like to do it right after records are loaded from db. Is it possible? How?

2

There are a couple different ways you could do it, depending on the use case. Have you considered a calculation? This would let you do it on demand when you need it.

calculate :thing_in_time_zone, :datetime, YourCalculation

Then you could load that calculation when making queries using Ash.Query.load or later using Api.load. If you must do it on the query you can use a preparation + an after_action hook to do it on all reads.

preparations do
  prepare fn query, _ -> 
    Ash.Query.after_action(query, fn query, results -> 
      Enum.map(results, &do_whatever_you_want/1)
    end)
  end
end
3

Also, I’m not sure what Ash.Transform is BTW, I don’t think we have anything by that name.

4

Thanks Zach.
Trying the second option, right now.
Is there any documentation related to preparations?

5

There is some frustratingly out of date bits here: Ash.Resource.Preparation — ash v3.29.3

Ultimately a preparation can look one of two ways:

prepare fn query, context -> 
  ...
end
# or
prepare Module
prepare {Module, foo: :bar} # or with opts

defmodule Module do
  use Ash.Resource.Preparation

  def prepare(query, opts, context) do
    query
  end
end

The actions guide talks more about the lifefcycle of each action type. Actions — ash v3.29.3

6

Thanks, Zach. That worked.