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

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.