How to update struct in nested embeds_many array

I’m finding it very difficult to figure out how to update an embedded struct that is in a nested embeds_many array.

Here’s the basic schema..

(Main row in postgres) Gathering → Activities (embeds_many Activity on Gathering) → rsvp (embeds_many ActivityRsvp on Activity)

The struggle to update the records in the rsvp collection is real. I’ve spent days reading methods to do this.

Ultimately, the insert works fine, but I can’t figure out how to make put_embed update the record.

A key difficulty was (which I believe is a bug), autogenerate_id was not working for the nested embeds_many, so I had to generate my own primary keys. This leads me to believe that this feature simply isn’t supported?

So to recap, if I bypass the error by generating my own primary key I can get insert to work fine using rsvps ++ [rcs], but I can’t get updates to work. Depending on the hack I try, I get a cornucopia of different errors. usually around get_change/6

Models:

  schema "gatherings" do
    embeds_many :activities, Activity
  end

 # Activity
  embedded_schema do
    embeds_many :rsvp, ActivityRsvp
  end

# ActivityRsvp
  embedded_schema do
    field :username, :string
    field :attending, :boolean
  end

  def change_rsvp_status_for_user(%Gathering{} = gathering, %Activity{ rsvp: rsvps } = activity, %{ username: username, attending: attending } = rsvp_params) do
    rcs = case get_rsvp_status_for_user(activity, username) do
      {:unknown, changeset} -> changeset
      {:ok, %ActivityRsvp{id: id} = r, _attendance_list} -> ActivityRsvp.changeset(r, rsvp_params)
    end

    case rcs.valid? do
     true -> acs = activity
             |> Activity.changeset
             |> put_embed(:rsvp,  [rcs])
             {:ok, a} = update_activity(gathering, acs )
             get_rsvp_status_for_user(a, username)
     false -> {:error, rcs}
    end
  end

 test/geeks/activities_test.exs:69
 ** (Ecto.NoPrimaryKeyValueError) struct `%ActivityRsvp{attending: true, comment: nil, **id: nil,** inserted_at: nil, no_show: false, updated_at: nil, username: "greybeard_prime"}` is missing primary key value
 stacktrace:
   (ecto) lib/ecto/embedded.ex:155: anonymous fn/3 in Ecto.Embedded.autogenerate_id/5