errors: [father: "is invalid"]
I think this is the culprit, there errors should be in the form:
errors: [field: {message, metadata}]
so in this case:
errors: [father: {"is invalid", []}]
Are you setting the is invalid error yourself, or is Ecto setting it (via cast_assoc etc)? If the latter, it seems it’s an Ecto bug. What’s your Ecto version?
btw, you’re defaulting father to tuple {}, you probably meant to write %{}, however for embeds/associations nil is probably better default. In any case, it’s still curious why you’re getting the error.
I got it working by puttin nil insteand of {}. Coming from javascript it’s still hard thinking in maps.
The error is sent by Ecto though, so i assume it’s an Ecto bug
Anyway, thank you both for your help 
ps : my ecto version is {:phoenix_ecto, "~> 3.2"}
glad it’s fixed!
my ecto version is
{:phoenix_ecto, "~> 3.2"}
could you paste your ecto version from mix.lock? (or run: mix deps | grep ecto :)) If you happen to run on a recent version there’s likely still a bug in ecto.
Aaaand… it’s fixed! See Handle relation changeset error in one place by qcam · Pull Request #2725 · elixir-ecto/ecto · GitHub by @qcam.