Ecto Changeset allowing a virtual association during the changeset isn't working

Question

I want a virtual assoc, so that I can do polymorphic assocs. So I have a picture schema, that is related to Users (and others). I will have a place in the callback where I assign the polymorphic type/id to the schema, but in order to do so, it needs to be assigned to the changes, but with this I'm struggled. Currently I have.

defmodule App.Picture do
  schema "pictures" do
    ...
    field :pictureable, :map, virtual: true
  end

  @attrs [:picture, :pictureable_id, :pictureable_type]

  def changeset(picture, attrs) do
    picture
    |> cast(attrs, @attrs ++ [:pictureable])
    |> handle_polymorphic
    |> cast_attachments(attrs, [:picture])
    |> validate_required(@attrs)
  end

  def handle_polymorphic(%{changes: %{pictureable: %type{id: id}}} = ch) do
    ch
    |> put_change(:pictureable_type, type)
    |> put_change(:pictureable_id, id)
  end
end

Fairly straightforward. You create a picture using attrs like %{picture: Path.to("/image"), pictureable: user}, which would be transformed into, %{picture: Arc.stuff, pictureable_id: 1, pictureable_type: "App.User"}

The issue with the code above is that :pictureable doesn't get kept in the changeset

pic = %Plug.Upload{content_type: ...}
user = insert(:user)
args = %{picture: img, pictureable: user}
ch = Picture.changeset(%Picture{}, args)

My changeset comes back like this

#Ecto.Changeset<
  action: nil,
  changes: %{
    picture: %{
      file_name: "10x10.png",
      updated_at: #Ecto.DateTime<2018-05-05 18:59:18>
    }
  },
  errors: [
    pictureable_id: {"can't be blank", [validation: :required]},
    pictureable_type: {"can't be blank", [validation: :required]},
    pictureable: {"is invalid", [type: :map, validation: :cast]}
  ],
  data: #App.Picture<>,
  valid?: false
>

Why will it not assign the map as expected for the virtual schema?

EDIT: It works if I use field :pictureable, :any, virtual: true. So it seems it's some issue with a struct not being a map, even though the user would pass a is_map(user) test. Any idea why it's being treated like it's not a map?

Lanny Bose Lanny Bose · Accepted Answer · 2018-05-16T03:01:41

My initial inclination is to just not cast the picturable at all since you really don't want Ecto to muck with the data type...

def changeset(picture, attrs) do
  picture
  |> cast(attrs, @attrs ++ [:pictureable])
  |> handle_polymorphic(attrs[:picturable])
  |> cast_attachments(attrs, [:picture])
  |> validate_required(@attrs)
end

def handle_polymorphic(ch, nil), do: ch
def handle_polymorphic(ch, %type{id: id}) do
  ch
  |> put_change(:pictureable_type, type)
  |> put_change(:pictureable_id, id)
end

Ecto Changeset allowing a virtual association during the changeset isn't working

1 Answers