Specify translatable attributes

[crbelaus]

When adding convencience functions to a translatable model (by using Trans module) it would be nice to explictly specify which attributes are translatable, and enforce the existence of those attributes in the changeset.

I am thinking about something like this (observe the fields opt being passed to use Trans)

defmodule Trans.Article do
  use Ecto.Schema
  use Trans, fields: [:title, :body], container: :translations

  import Ecto.Changeset

  ...
end

About the validation, I am thinking about adding a new function to the model that checks if provided translations match the specified fields passed before. This function should be called explicitly on the desired changeset.

[crbelaus]

If we have translatable attributes specified on compile time, some interesting posibilities emerge:

• We could add convenience functions to retrieve each attribute. For example, if we translate the attribute title we could add a Article.title/2 function which would receive an article and a locale, and return the title of the article translated in the given locale (this would be idiomatic Ruby, but I suspect that it would not be idiomatic Elixir)
• We can enforce attributes on Trans.Translatable.translate/3 function. If we only translate title attribute, and are being asked for the fake attribute we could return an informative error.

[crbelaus]

We will use a translates key, which contains the list of translatable attributes for the model. An optional defaults key can also be specified and will contain the default options that will be automatically passed to the convenience functions.