traverse_errors/2 doesn't seem to work when run on a top-level schema which has_many children schemas that have polymorphic embeds

[x-ji]

An example:

defmodule A.Parent do
  # ...
  schema "parents" do
    has_many(:children, A.Child)
  end
  
  def changeset(%__MODULE__{} = struct, attrs) do
    struct
    |> cast_assoc(:children)
  end
end

defmodule A.Child do
  schema "children" do
    # ...

    polymorphic_embeds_one(:something, )
  end
end

params = %{
  # ...
  children: [
    %{
      something: ... # Here contains an error
    },
    %{
      something: ...
    }
  ]
}

When a changeset is inserted directly at the Parent level, but contains errors in the polymorphic embed of something at the Child level, PolymorphicEmbed.traverse_errors/2 when run on the resulting changeset still returns an empty map.

Not sure if Ecto.Changeset.traverse_errors/2 has the same behavior. At least the documentation of the Ecto function says

Traverses changeset errors and applies the given function to error messages.

This function is particularly useful when associations and embeds are cast in the changeset as it will traverse all associations and embeds and place all errors in a series of nested maps.

[mathieuprog]

Can you test if the behavior is different? Replace the polymorphic embed by embeds_one and look for differences?

[mathieuprog]

Fixed traverse_errors.