I’m trying to create a form which has a list of nested items, each of which contain some union field. I have the following hierarchy: WorkoutTemplate has many SetGroupTemplate where SetGroupTemplate looks like the following module and both FixedReps and ClassicOverload are embedded ash resources

defmodule Weightroom.Template.SetGroupTemplate do
  @moduledoc false

  use Ash.Resource,
    domain: Weightroom.Template,
    data_layer: AshPostgres.DataLayer

  alias Weightroom.Template.Progression

  attributes do
    uuid_primary_key :id
    attribute :order, :integer, public?: true, allow_nil?: false

    attribute :progression, :union do
      public? true
      allow_nil? false

      constraints types: [
                    fixed_reps: [
                      type: Progression.FixedReps,
                      tag: :type,
                      tag_value: :fixed_reps
                    ],
                    classic_overload: [
                      type: Progression.ClassicOverload,
                      tag: :type,
                      tag_value: :classic_overload
                    ]
                  ]
    end
  end

  relationships do
    belongs_to :workout, Weightroom.Template.WorkoutTemplate do
      allow_nil? false
    end

    belongs_to :exercise, Weightroom.Journal.Exercise do
      allow_nil? false
      public? true
    end
  end

  postgres do
    table "set_group_templates"
    repo Weightroom.Repo

    references do
      reference :workout, on_delete: :delete
      reference :exercise, on_delete: :delete
    end

    custom_statements do
      statement :unique_order do
        up ~s|ALTER TABLE set_group_templates ADD CONSTRAINT set_group_templates_unique_order_index UNIQUE (workout_id, "order") DEFERRABLE INITIALLY DEFERRED|

        down "ALTER TABLE set_group_templates DROP CONSTRAINT set_group_templates_unique_order_index"
      end
    end
  end

  actions do
    defaults [:read, :destroy, create: :*]
  end

When I create the form and try to add a new SetGroupTemplate I get the following error

    template
    |> AshPhoenix.Form.for_update(:update, forms: [auto?: true])
    |> AshPhoenix.Form.add_form("form[set_groups]",
      params: %{
        "progression" => %{
          "_union_type" => "fixed_reps"
        }
      }
    )

** (RuntimeError) Got no "_union_type" parameter, and no union type had a tag & tag_value pair matching the params.

If you are adding a form, select a type using `params: %{"_union_type" => "type_name"}`, or if one
or more of your types is using a tag you can set that tag with `params: %{"tag" => "tag_value"}`.

Params:

%{}

Available types:

[
  classic_overload: [
    type: Weightroom.Template.Progression.ClassicOverload,
    constraints: [on_update: :update_on_match],
    tag: :type,
    tag_value: :classic_overload
  ],
  fixed_reps: [
    type: Weightroom.Template.Progression.FixedReps,
    constraints: [on_update: :update_on_match],
    tag: :type,
    tag_value: :fixed_reps
  ]
]

If I instead use “type” instead of “_union_type” as the key for the new progression I get the same error. Stepping through the code of ash authentication it looks like AshPhoenix.Form.Auto.determine_type/3 is called twice. The first time with the map of params and it doesn’t raise. The second time, params is empty and the function raises.

I saw there is a test for union forms here and I can’t see what I’m doing differently other than using an embedded resource for the union type instead of a simple type like string/int/etc. Any help understanding where I’m going wrong would be appreciated.