just trying to preload an association

 p = Pix |> Repo.get("074a1581-fd36-49e7-a756-03d30f378312")

%PolymorphicProductions.Social.Pix{
  __meta__: #Ecto.Schema.Metadata<:loaded, "pics">,
  asset: "https://d1sv288qkuffrb.cloudfront.net/polymorphic-productions/photos/37409259750_420cebcc8a_b.jpg",
  asset_preview: "https://d1sv288qkuffrb.cloudfront.net/polymorphic-productions/photos/preview/37409259750_420cebcc8a_b.jpg",
  comments: #Ecto.Association.NotLoaded<association :comments is not loaded>,
  description: "asdf",
  id: "074a1581-fd36-49e7-a756-03d30f378312",
  inserted_at: ~N[2018-11-04 18:58:03.347422],
  photo: nil,
  updated_at: ~N[2018-11-04 18:58:03.347444]
}
iex(10)> p |> Repo.preload([:comments])
** (Ecto.Query.CastError) deps/ecto/lib/ecto/association.ex:516: value `"074a1581-fd36-49e7-a756-03d30f378312"` in `where` cannot be cast to type :id in query:

from c in PolymorphicProductions.Social.Comment,
  where: c.pix_id == ^"074a1581-fd36-49e7-a756-03d30f378312",
  order_by: [asc: c.pix_id],
  select: {c.pix_id, c}

    (elixir) lib/enum.ex:1925: Enum."-reduce/3-lists^foldl/2-0-"/3
    (elixir) lib/enum.ex:1418: Enum."-map_reduce/3-lists^mapfoldl/2-0-"/3
    (elixir) lib/enum.ex:1925: Enum."-reduce/3-lists^foldl/2-0-"/3
    (ecto) lib/ecto/repo/queryable.ex:124: Ecto.Repo.Queryable.execute/5
    (ecto) lib/ecto/repo/queryable.ex:37: Ecto.Repo.Queryable.all/4
    (elixir) lib/enum.ex:1314: Enum."-map/2-lists^map/1-0-"/2

Any ideas what I’m doing wrong?