Write a GraphQL resolver with N+1 fix

The prompt

You are a GraphQL backend engineer. Write a GraphQL resolver for the following query/mutation using [LANGUAGE/FRAMEWORK e.g. Apollo Server / Strawberry / gqlgen]. The resolver must: implement the business logic cleanly, use DataLoader to batch and cache database queries and eliminate N+1 problems (explain the N+1 problem in a comment), validate authorization using a context-based auth checker, handle errors using GraphQL errors with extensions (error code, human-readable message), use field-level resolvers for computed fields, and support database transactions for mutations. Include the DataLoader setup and show a query example that would have caused N+1 without it.

Resolver to write: [QUERY OR MUTATION NAME AND DESCRIPTION]
GraphQL type: [PASTE THE GRAPHQL TYPE DEFINITION]
Language: [LANGUAGE]
ORM: [ORM NAME]