如何使用Ecto Changeset为Schema添加字段互斥约束？

Absolutely! You can cleanly implement this exact requirement — where exactly one of a or b must be provided (no both, no neither) alongside a mandatory c field — using a custom Ecto validation function. This keeps your changeset logic readable and reusable.

Here's the complete, working implementation:

defmodule MyApp.Foo do
  use Ecto.Schema
  import Ecto.Changeset

  schema "foo" do
    field(:a, :string)
    field(:b, :string)
    field(:c, :string)
    timestamps()
  end

  @doc false
  def changeset(foo, attrs) do
    foo
    |> cast(attrs, [:a, :b, :c])
    |> validate_required([:c], message: "is required")
    |> validate_mutual_exclusion([:a, :b])
  end

  defp validate_mutual_exclusion(changeset, [field1, field2]) do
    # Get values, accounting for both existing struct values and new changes
    value1 = get_field(changeset, field1)
    value2 = get_field(changeset, field2)

    cond do
      # Both fields have non-empty values: invalid
      not is_nil(value1) and value1 != "" and not is_nil(value2) and value2 != "" ->
        changeset
        |> add_error(field1, "cannot be used together with #{field2}")
        |> add_error(field2, "cannot be used together with #{field1}")

      # Neither field has a non-empty value: invalid
      (is_nil(value1) or value1 == "") and (is_nil(value2) or value2 == "") ->
        changeset
        |> add_error(field1, "either #{field1} or #{field2} must be provided")
        |> add_error(field2, "either #{field1} or #{field2} must be provided")

      # Exactly one field has a value: valid
      true ->
        changeset
    end
  end
end

Key Details Explained: #

• Custom Validation Function: validate_mutual_exclusion/2 is a private helper that handles the core logic. It’s reusable across any other changesets that need this "exactly one of two fields" rule.
• Value Checking: We use get_field/2 instead of fetch_change/2 because we need to account for both new values from the attrs and existing values on the struct (if you’re updating an existing record).
• Conditional Logic:
  • Rejects cases where both a and b are filled in, adding clear errors to both fields.
  • Rejects cases where neither a nor b are filled in, again adding clear errors.
  • Passes through changesets where exactly one of the fields is present.

Adaptations for Other Field Types: #

If a or b are non-string types (e.g., integers, booleans), adjust the value checks to match:

• For integers: Replace value1 != "" with value1 != 0 (or whatever your "empty" value is).
• For booleans: Just check not is_nil(value1) since booleans are either true, false, or nil.

内容的提问来源于stack exchange，提问作者jonnystoten