Elixir 验证三分之一所需的_

我有

Xyz

，它要么有

国家id

，要么有

联邦州id

，要么有

城市id

。但其中只有一个（不是全部三个，也不是两个）。我如何对此进行验证？另外，我如何才能仅对该字段执行

关联约束/1

defmodule Example.Location.Xyz do
  use Ecto.Schema
  import Ecto.Changeset
  alias Example.Location.Xyz

  schema "xyzs" do
    field :name, :string
    belongs_to :country, Example.Location.Country
    belongs_to :federal_state, Example.Location.FederalState
    belongs_to :city, Example.Location.City

    timestamps()
  end

  @doc false
  def changeset(%Xyz{} = xyz, attrs) do
    school
    |> cast(attrs, [:name, :country_id, :federal_state_id, :city_id])
    |> validate_required([:name, :country_id, :federal_state_id, :city_id])
    |> assoc_constraint(:country)
    |> assoc_constraint(:federal_state)
    |> assoc_constraint(:city)
  end
end

---

我将创建一个函数来检查3个字段中是否有1个字段存在，并添加正确的

assoc\u约束

：

@doc false
def changeset(%Xyz{} = xyz, attrs) do
  school
  |> cast(attrs, [:name, :country_id, :federal_state_id, :city_id])
  |> validate_required([:name])
  |> validate_one_of_present([:country_id, :federal_state_id, :city_id])
end

def validate_one_of_present(changeset, fields) do
  fields
  |> Enum.filter(fn field ->
    # Checks if a field is "present".
    # The logic is copied from `validate_required` in Ecto.
    case get_field(changeset, field) do
      nil -> false
      binary when is_binary(binary) -> String.trim_leading(binary) == ""
      _ -> true
    end
  end)
  |> case do
    # Exactly one field was present.
    [field] ->
      without_id = field |> Atom.to_string |> String.replace_suffix("_id", "") |> String.to_existing_atom
      assoc_constraint(changeset, without_id)
    # Zero or more than one fields were present.
    _ ->
      add_error(changeset, hd(fields), "expected exactly one of #{inspect(fields)} to be present")
  end
end

---

代码未经测试，如果发现任何错误，请告诉我

哇，我不知道你能在案件陈述中吹毛求疵。但这绝对有道理。