docs(mercurius-js#28): example with custom directive

examples/withCustomDirectives.js

@@ -0,0 +1,83 @@
+'use strict'
+
+const Fastify = require('fastify')
+const { mapSchema, getDirective, MapperKind, printSchemaWithDirectives, getResolversFromSchema } = require('@graphql-tools/utils')
+const { mergeResolvers } = require('@graphql-tools/merge')
+const { makeExecutableSchema } = require('@graphql-tools/schema')
+const mercurius = require('mercurius')
+const { buildFederationSchema } = require('../')
+
+const users = {
+  1: {
+    id: '1',
+    name: 'John',
+    username: '@john'
+  },
+  2: {
+    id: '2',
+    name: 'Jane',
+    username: '@jane'
+  }
+}
+
+const upperCaseDirectiveTypeDefs = 'directive @upper on FIELD_DEFINITION'
+const uppercaseTransformer = (schema) =>
+  mapSchema(schema, {
+    [MapperKind.FIELD]: (fieldConfig) => {
+      const upperDirective = getDirective(schema, fieldConfig, 'upper')?.[0]
+      if (upperDirective) {
+        fieldConfig.resolve = async (obj, _args, _ctx, info) => {
+          const value = obj[info.fieldName]
+          return typeof value === 'string' ? value.toUpperCase() : value
+        }
+      }
+    }
+  })
+
+const app = Fastify()
+const schema = `
+  ${upperCaseDirectiveTypeDefs}
+
+  extend type Query {
+    me: User
+  }
+
+  type User @key(fields: "id") {
+    id: ID! 
+    name: String @upper
+    username: String
+  }
+`
+
+const resolvers = {
+  Query: {
+    me: () => {
+      return users['1']
+    }
+  },
+  User: {
+    __resolveReference: source => {
+      return users[source.id]
+    }
+  }
+}
+
+const federationSchema = buildFederationSchema(schema)
+
+const executableSchema = makeExecutableSchema({
+  typeDefs: printSchemaWithDirectives(federationSchema),
+  resolvers: mergeResolvers([getResolversFromSchema(federationSchema), resolvers])
+})
+
+app.register(mercurius, {
+  schema: executableSchema,
+  schemaTransforms: [uppercaseTransformer],
+  graphiql: true
+})
+
+app.get('/', async function () {
+  const query = '{ _service { sdl } }'
+  return app.graphql(query)
+})
+
+app.listen({ port: 3000 })