`:any` type for embedded schemas in non-virtual fields

Hey there!

We are using Ecto to parse some API response (it works pretty well!).
We want this response to be converted to a struct, so we first do all the needed validations and at the end we do an apply_action(changeset, :parse).
To do so, we would like to have a module that defines an embedded_schema like the following (we won’t save this):

defmodule MyApp.Response do
  use Ecto.Schema

  @primary_key false
  embedded_schema do
    field(:custom_field, :any)
    ...
  end
end

The “problem” comes with this :any type. When compiling, Ecto complains that only virtual fields can be defined with the :any type.
Here the error:

== Compilation error in file lib/my_app/api/response.ex ==
** (ArgumentError) only virtual fields can have type :any, invalid type for field :custom_field
    (ecto 3.11.1) lib/ecto/schema.ex:2012: Ecto.Schema.__field__/4
    lib/my_app/api/response.ex:36: (module)
    (ecto 3.11.1) lib/ecto/schema.ex:2241: Ecto.Schema.__embeds_module__/4
    lib/my_app/api/response.ex:32: (module)

Is there an explanation of why they must be virtual?

In our use case, this field can have different types (boolean, integer, string, etc.), so we need this flexibility here.

We think that, as embedded_schemas are just saved as blobs or, like in our case, not saved at all, this doesn’t make a lot of sense. But maybe someone has a better understanding about this.

Thanks in advance!