ecto_validations

parent.ex

defmodule Parent do
  use Ecto.Schema
  import Ecto.Changeset

  schema "parents" do
    field :name, :string

    embeds_one :child, Child do
      field :name, :string
      field :age,  :integer
    end
  end

  def changeset(schema, params) do
    schema
    |> cast(params, [:name])
    |> cast_embed(:child, with: &child_changeset/2)
  end

  defp child_changeset(schema, params) do
    schema
    |> cast(params, [:name, :age])
  end

  # #Ecto.Changeset<
  # action: nil,
  # changes: %{
  #   child: #Ecto.Changeset<
  #     action: :insert,
  #     changes: %{age: 3, name: "Jack"},
  #     errors: [],
  #     data: #Parent.Child<>,
  #     valid?: true
  #   >,
  #   name: "Charlie"
  # },
  # errors: [],
  # data: #Parent<>,
  # valid?: true
  # >
  def test do
    new_parent = %{
      name: "Charlie",
      child: %{
        name: "Jack",
        age: 3
      }
    }

    Parent.changeset(%Parent{}, new_parent)
  end

  # ** (Ecto.CastError) expected params to be a :map, got: `[%{age: 3, name: "Jack"}]`
  #   (ecto 3.5.7) lib/ecto/changeset.ex:514: Ecto.Changeset.cast/6
  #   (ecto 3.5.7) lib/ecto/changeset/relation.ex:129: Ecto.Changeset.Relation.do_cast/6
  #   (ecto 3.5.7) lib/ecto/changeset/relation.ex:318: Ecto.Changeset.Relation.single_change/5
  #   (ecto 3.5.7) lib/ecto/changeset/relation.ex:113: Ecto.Changeset.Relation.cast/5
  #   (ecto 3.5.7) lib/ecto/changeset.ex:811: Ecto.Changeset.cast_relation/4
  def test_bad_input do
    new_parent = %{
      name: "Charlie",
      child: [%{
        name: "Jack",
        age: 3
      }]
    }

    Parent.changeset(%Parent{}, new_parent)
  end
end

In this scenario, we are attempting to use Ecto as an "input validator". Pretend that the input objects were just deserialized user input.

It feels like Ecto should be handling this second scenario by returning an Ecto.Changeset object with validation errors rather than throwing...

Why would you expect that? You've said it embeds_one not that it embeds_many so it makes sense that it would blow up on a list in the second example.