@requires

When extending a schema, it's often because you want to expose data from Node.js that would be too difficult (or impossible) to access from PostgreSQL

• in other words, we come to a situation where we don't want to rely on Postgres to process and give us data, but instead want to rely on Node to do the job for us.

USD to CAD example

• Imagine in our database we store the price of a product in USD. However, we want to be able to query the priceInCad, directly as a field on the graphql schema:

query Product {
	product {
		priceInCad
	}
}

We can accomplish this by making a custom Postgraphile plugin, which would extend the Graphql schema to include the priceInCadCents field on the Product type:

const MyForeignExchangePlugin = makeExtendSchemaPlugin(build => {
  return {
    typeDefs: gql`
			extend type Product {
				priceInCadCents: Int! @requires(columns: ["price_in_us_cents"])
			}`,
    resolvers: {
      Product: {
        priceInCadCents: async product => {
          // Note that the columns are converted to fields, so the case changes
          // from `price_in_us_cents` to `priceInUsCents`
          const { priceInUsCents } = product;
          return await convertUsdToCad(priceInUsCents);
        },
      },
    },
  };
});

we include the @requires directive to show that in order to calculate this field, we need the postgres column price_in_us_cents