How to resolve nested types in GraphQL?

I'm having trouble resolving graphql nested types. I can successfully get the UserMetrics nested resolver to fire, but the parent resolver object (user) is null. Am I misunderstanding the GraphQL resolver map?

Schema:

type User {
    id: String!
    metrics: UserMetrics
}

type UserMetrics {
    lastLogin: String!
}

Resolver:

Query: {
    user(_, { id }, ctx) {
        return { id }; 
    }
},

User: {
    metrics(): ({}), // UserMetrics.lastLogin doesn't fire without this
},

UserMetrics: {
    lastLogin(user) {
        console.log(user); // null
    }
},