How can I generate values in create actions in Ash?

I have this action, which is not working the way I expected.

create :build do
  accept [:identity_id]
  change set_attribute(:token, Helpers.rand_token())
  change set_attribute(:inserted_at, DateTime.utc_now())
end

token and inserted_at get a fixed value from when the code was evaluated, if I’m not mistaken. But I want to, of course, generate a new value every time the action runs.

I figured out I could accomplish it this way:

create :build do
	accept \[:identity\_id]

	change fn changeset, \_ ->
	 	changeset
		|> Ash.Changeset.change\_attribute(:token, Helpers.rand\_token())
		|> Ash.Changeset.change\_attribute(:inserted\_at, DateTime.utc\_now())
	end
end

It feels a bit clunky. Is there a more straightforward way? I tried to pass a thunk instead of the value directly to set_attribute, but it doesn’t seem to work.

Or are there other ways to do this in a more idiomatic way? To generate values when a resource is created, I mean.

Since both of the functions are zero-arity, you can do this

create :build do
  accept [:identity_id]
  change set_attribute(:token, &Helpers.rand_token/0)
  change set_attribute(:inserted_at, &DateTime.utc_now/0)
end

If you provide a zero-arity function, it will be called, and the value will be set. Otherwise, you must do the anonymous function route or create a change module.

Also, for :inserted_at, you can do create_timestamp :inserted_at in the attributes section instead of a manual change. Attributes — ash v3.4.57

2 Likes

Nice! Thanks!

I was bitten by my lack of Elixir skills.

I tried change set_attribute(:token, Helpers.rand_token) and change set_attribute(:token, fn -> Helpers.rand_token() end). Neither worked.

Now I need to learn why I must capture the function, and why Helpers.rand_token or even Helpers.rand_token/0 differs from &Helpers.rand_token/0. I suppose it’s because of the different scopes for variables and functions.

I’m surprised the set_attribute(:token, fn -> Helpers.rand_token() end) didn’t work. I’ll ask on the discord and let you know.

If you aren’t on Discord, it’s a great place to learn Ash

Ok, thanks again.

I’ll join Discord.