Detect empty_values inside lists when casting.

[marcandre]

See https://elixirforum.com/t/phoenix-casting-multiple-checkboxes-to-array-ecto-enum/46252/6

This way, an HTML form with checkboxes having names ending with [] can have
a hidden input with empty value to ensure the presence of that parameter
if none of the checkboxes are checked.

[josevalim]

💚 💙 💜 💛 ❤️

[leandrocp]

Hi @marcandre and @josevalim I just found out this PR has caused a breaking change on our app because oddly enough we were relying on the old behavior, ie: we expect the changeset to keep "" 😆

It's kinda an easy fix on our side and the change is welcome, I'm just wondering if it's worth adding a note on CHANGELOG.md that it's potentially a breaking change, something like:

[Ecto.Changeset] Detect empty_values inside lists when casting. Note it may be a breaking chance if you expect the value "" to be returned.

[marcandre]

Sounds like a good idea. I'm trying to double check if it was included in version 3.7.2, but that commit (960725b) "does not belong to any branch on this repository"...

Ah, done as d382713

[fuelen]

Another breaking change that I've noticed is that this code now doesn't put the error "can't be blank" into changeset:

{%{}, %{uris: {:array, :string}}}
|> Ecto.Changeset.cast(%{uris: []}, [:uris], empty_values: ["", []])
|> Ecto.Changeset.validate_required(:uris)

but it was there in Ecto. 3.7.2