How to reference a specific ENUM value in GraphQL type?

Question

I have the following schema:

enum PaymentTypeName {
  PAYMENT_CARD
  PAYMENT_CARD_TOKEN
}

interface Payment {
  id: ID!
  type: PaymentTypeName!
}

type PaymentCardPayment implements Payment {
  id: ID!
  type: PaymentTypeName!
  card: PaymentCard!
}

type PaymentCardTokenPayment implements Payment {
  id: ID!
  type: PaymentTypeName!
  card: PaymentCard!
}

When Payment is PaymentCardPayment or PaymentCardTokenPayment is determined by the value of type, i.e. it is either PAYMENT_CARD or PAYMENT_CARD_TOKEN.

How do I signify in the interface, that PaymentCardPayment/ PaymentCardTokenPayment inherit a specific value of PaymentTypeName?

I have tried various combinations of:

type PaymentCardPayment implements Payment {
  id: ID!
  type: PaymentTypeName.PAYMENT_CARD!
  card: PaymentCard!
}

and:

type PaymentCardPayment implements Payment {
  id: ID!
  type: PaymentTypeName[PAYMENT_CARD]!
  card: PaymentCard!
}

but all of these prompt a syntax error and I was unable to find the relevant documentation.

Answer 1

What you're trying to do is not supported in GraphQL. If a field's type is declared to be PaymentTypeName, and PAYMENT_CARD and PAYMENT_CARD_TOKEN are both valid values of PaymentTypeName, then they must also be valid values for that field. There is no way to take an existing type9 whether it's an enum, scalar, or object type) and conditionally create a subset of possible values from the set of possible values already defined by the type.

That said, if PaymentCardPayment.type will always resolve to PAYMENT_CARD and PaymentCardTokenPayment.type will always resolve to PAYMENT_CARD_TOKEN, then it doesn't really make sense to use an enum here at all. In fact, in this specific case, we can omit the type field entirely. After all, the only purpose of the field in this case is to allow the client to distinguish between the possible types that Payment may resolve to. However, GraphQL already provides us with a __typename field that does just that by resolving to the name of the resolved type.

So in this specific instance, it's sufficient to just do:

interface Payment {
  id: ID!
}

type PaymentCardPayment implements Payment {
  id: ID!
  card: PaymentCard!
}

type PaymentCardTokenPayment implements Payment {
  id: ID!
  card: PaymentCard!
}

and query a field whose type is Payment like this:

{
  payments {
    id
    __typename
    ... on PaymentCardPayment {
      card {
        # ...
      }
    }
    ... on PaymentCardTokenPayment {
      card {
        # ...
      }
    }
  }
}

Answer 2

You are trying to declare the field value in your type schema, which is not what a schema is meant for. You should only be declaring your field type within your schema, in this case it is just type: PaymentTypeName. You have it correct in your first code block.

Your PaymentCardPayment's type resolver function should return the value of the enum, in your case, PAYMENT_CARD.

Your PaymentCardTokenPayment's type resolver function should return the value of PAYMENT_CARD_TOKEN.