How do I use Ash.Transform

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

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

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

There is some frustratingly out of date bits here: Ash.Resource.Preparation — ash v3.29.3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

Thanks, Zach. That worked.