Union (聯集)

Union

只有字串字面值的 union 收斂成單一 enum，形狀與字串 enum 相同：

model Sub {
  status: "active" | "canceled" | "paused";
}

Sub:
  type: object
  properties:
    status:
      type: string
      enum:
        - active
        - canceled
        - paused
  required:
    - status

其他 union 轉成 anyOf，一個 variant 一個分支。T | null 就是帶 null 分支的 union。JSON Schema draft-07 沒有 nullable 關鍵字：

union PaymentMethod {
  card: CreditCard,
  transfer: BankTransfer,
}

model Payment {
  method: PaymentMethod;
  memo: string | null;
}

PaymentMethod:
  anyOf:
    - $ref: "#/components/schemas/CreditCard"
    - $ref: "#/components/schemas/BankTransfer"
Payment:
  type: object
  properties:
    method:
      $ref: "#/components/schemas/PaymentMethod"
    memo:
      anyOf:
        - type: string
        - type: "null"
  required:
    - method
    - memo

若要恰好一個分支成立（而非「至少一個」），在 union 標 @oneOf：

@oneOf
union Shape {
  circle: Circle,
  square: Square,
}

Shape:
  oneOf:
    - $ref: "#/components/schemas/Circle"
    - $ref: "#/components/schemas/Square"