Ah, I see what you mean. Using multiple explicit forms like that only really helps with the form structure itself. You should first be looking at how to implement this logic in your actions, and then work on the form logic, which likely would not require those nested forms once you are ready. Unfortunately I don’t have time to go through your entire code and craft an answer for your specific case, but I can provide a basic example.

# on your user resource

create :register do
  argument :company_name, :string, allow_nil?: false

  change fn changeset, _ -> 
    Ash.Changeset.before_action(changeset, fn changeset, _ -> 
      org = create_organization(changeset.arguments.company_name)
      Ash.Changeset.force_change_attributes(changeset, :company_id, company.id)
    end)
  end
end

Using “hooks” (Ash.Changeset.before_action, Ash.Changeset.after_action, etc.) allow us to build progressively more complex multi-step actions. Additionally, we can use arguments to accept additional inputs that don’t map directly to attributes to be changed.

With that, you can create a single form with all the inputs you need, no need to define nested forms.

Using manage_relationship

Given that what you are doing is essentially creating related data as part of an action, there is a builtin that can help with this, called manage_relationship.

You can use it like so:

create :register do
  argument :personal_information, :map, allow_nil?: false
  argument :company_information, :map, allow_nil?: false

  change manage_relationship(:personal_information, type: :create)
  change manage_relationship(:personal_information, type: :create)
end

Then, when creating your form, you can use auto?: true, which will automatically detect the necessary nested forms. The two in combination should be all you need to accomplish your stated goal.

AshPhoenix.Form.for_create(
  Mgx.Accounts.User,
  :register_with_password,
  forms: [
    auto?: true
  ]
)
|> AshPhoenix.Form.add_form([:personal_information])
|> AshPhoenix.Form.add_form([:company_information])
|> to_form()