How to change return type for createFieldExtension

[herecydev]

Given the following schemaCustomization:

exports.createSchemaCustomization = ({ actions }) => {
  const { createFieldExtension, createTypes } = actions;

  createFieldExtension({
    name: "currencyFormat",
    description: `Add currency formatting options`,
    args: {
      formatCurrency: `Boolean`,
    },
    extend(options, fieldConfig) {
      return {
        ...fieldConfig.args,
        args: {
          formatCurrency: {
            type: `Boolean`,
            description: `Formats price to a currency string`
          },
        },
        async resolve(source, args, context, info) {
          const resolver = fieldConfig.resolve || context.defaultFieldResolver
          const price = await resolver(source, args, context, info)
          return args.formatCurrency ? "some_formatted_string" : price
        },
      }
    },
  })

  createTypes(schema);
};

and the following schema:

type SomeType implements Node @dontInfer {
    price: Float! @currencyFormat
  }

I want to be able to query the price field, so that both the following are possible:

{
  price //just returns the float price
}

{
  price(formatCurrency: true) //returns a string
}

I feel like this is exactly what Gatsby's @dateFormat directive is already doing when you pass it (formatString: "YYYY-MM-DD"). Have I done something wrong here? How can I get the extension to return different types correctly?

[vladar]

Hey @herecydev

Thanks for this question! The field type cannot change after it is defined in the schema. The only way you can do this is by setting a custom scalar type on your field. Gatsby includes the most general type: JSON (Date is another example but it's always resolves to string). Technically, you can introduce your own custom scalar type via schema customization (an example) but this is not documented and I would generally recommend staying away from custom scalar types even in regular GraphQL APIs.

[herecydev]

Thanks, I think I understand why the Date works then. If you're not recommending using the scalar Types; how would you best solve the problem then?

I've used customResolvers in some places (adding another field with a different return type that can consume the node's data). But there's no reusability, each field is manually added. What I'm after here is a way of the graphql consumer specifying the data should be transformed. It sounds exactly what directives are there for

[vladar]

There are a couple of options depending on the situation:

1. Have a Money type (with custom resolvers):

type Money {
  value: Float
  formatted(currency: CurrencyEnum): String
}

2. Have another field priceFormatted along with your price field.

3. And finally: use a JSON type or just a String type with a directive. Directives only work when they are compatible with the underlying field type.