Hey there! So, I think what you’re looking for are changes and potentially lifecycle hooks. Here is an example of setting an attribute based on other changes.

create :create do
  accept [:first_name, :last_name]

  change fn changeset, _ -> 
    first_name = Ash.Changeset.get_attribute(changeset, :first_name) || ""
    last_name = Ash.Changeset.get_attribute(changeset, :last_name) || ""
    Ash.Changeset.force_change_attribute(first_name <> " " <> last_name)
  end
end

If you want to do some “expensive” work, or work that should happen in the same transaction, you can use before_action and after_action hooks (probably before action in your case of affecting the attribute changes)

create :create do
  accept [:first_name, :last_name]

  change fn changeset, _ -> 
    Ash.Changeset.before_action(changeset, fn changeset -> 
      something = get_something_from_the_database()
      Ash.Changeset.force_change_attribute(changeset, :foo, something)
    end)
  end
end