graphql-example | Intuitive GraphQL Resolver Example - Application example | GraphQL library

You've defined resolvers for your queries, while are just fields on your Query type. However, you don't have any resolvers for the group field on the User type, so GraphQL falls back to using the default resolver for that field. Since the property field on the Object User resolves to is just an array of ids, GraphQL doesn't know how to make sense of it -- after all, you told it groups would be an array of objects.

You'll need to add a resolver for the groups field and transform that array into a Promise that will resolve to an array of Group objects: