Apply callback functions for schemas

[daniikpando]

Hi everyone!
We're trying to adapt this library in our elixir project, but we found a little customization problem.

Problem

The schemas are in the high layer level of the API and that is good, but sometimes we need to transform some values that were accepted in one format, and then we need to convert to another internal format, for example:

The IDs must be accepted in base62 format, but internally we need to work with it in UUID format (to make db query and etc...), it's not good to convert the IDs into the business logic because it increases the complexity of the code and we have to make it everytime. What happen if we try solving this by leveraging the serialization process with Open API spex.

Maybe it could be good if we implement callbacks function for schemas and the general idea could be to create a callback function that should be call in the cast and validate logic for every field that it's being validated, for example:

defmodule OpenApiSpex do
   ...
   defmacro schema(body, opts \\ []) do
      quote do
        ...
        if Map.get(@schema, :"x-struct") == __MODULE__ do
           ...
           @doc """
            Allows to transform a value after a field were validated.
            """
           @spec post_validate_field(field :: atom, value :: any) :: updated_value :: any
           def post_validate_field(_field, value), do: value
           defoverridable post_validate_field: 2
        end
    end
end

call post_validate_field in the OpenApiSpex.Cast functions (i'm not sure of this).

and for each module schema, we can override the post_validate_field to apply our custom transformations.

defmodule User do
    require OpenApiSpex
    
    OpenApiSpex.schema(%{
      title: "User",
      description: "A user of the app",
      type: :object,
      properties: %{
        id: %Schema{title: "Base62uuid",
                              type: :string,
                              pattern: ~r/^[0-9A-Za-z_-]{22}/,
                              description: "Base62 ID"},
        name: %Schema{type: :string, description: "User name", pattern: ~r/[a-zA-Z][a-zA-Z0-9_]+/},
        email: %Schema{type: :string, description: "Email address", format: :email},
        inserted_at: %Schema{
          type: :string,
          description: "Creation timestamp",
          format: :"date-time"
        },
        updated_at: %Schema{type: :string, description: "Update timestamp", format: :"date-time"}
      },
      required: [:name, :email],
      example: %{
        "id" => "1CKjowTbRoOvES9AOYGniA",
        "name" => "Joe User",
        "email" => "[email protected]",
        "birthday" => "1970-01-01T12:34:55Z",
        "inserted_at" => "2017-09-12T12:34:55Z",
        "updated_at" => "2017-09-13T10:11:12Z"
      }
    })

    def post_validate_field(:id, value), do: Base62UUID.decode!(value)
    def post_validate_field(_field, value), do: value
  end

Let me know if this could be possible or it's a crazy strange idea :)