type-graphql: How to know which fields are returned by Resolver

Question

I am writing a backend application that uses type-graphql. The GraphQL schema includes a Folder type that looks like this:

type Folder {
  id: ID!
  name: String!
  rules: [Rule!]!
  parent: Group
}

Rule and Group are other types.

My Resolver defines a Query called folders that returns all Folders:

import {Query, Resolver} from 'type-graphql'

@Resolver(() => Folder)
export default class FolderResolver {

  @Query(() => [Folder])
  folders(): Promise<Folder[]> {
    // query db and return results
  }
}

To query the folders, their rules, and their parent group from the database, I have to join the database tables and return the joined result, which works without problems. However: If the rules field and / or the parent field was not requested in the GraphQL query, I could omit the joins and make the database query much more efficient. So the fields that were requested in the GraphQL query have a definite effect on the logic that I need to execute, but I don't see a way of finding out which fields were requested inside the folders function.

When a Query in a type-graphql Resolver is executed, is there a way to determine which fields were requested in the GraphQL query?

Answer 1

Have a look at Field Resolver. The annotation is specifically made for such purpose

import {Query, Resolver, FieldResolver} from 'type-graphql'

@Resolver(() => Folder)
export default class FolderResolver {

  @FieldResolver()
  rules(@Root() folder: Folder) {
    // ...
  }


  @FieldResolver()
  parent(@Root() folder: Folder) {
    // ...
  }

  @Query(() => [Folder])
  folders(): Promise<Folder[]> {
    // query db and return results
  }
}

Now, when a user requests a query without rules or parent as a part of their attributes, they won't be called, if they do either or both will be called. Your logic to fetch data for them resides in their own field resolvers.

---

Updated as per comment:

If you have to get fields queries by the clients, you may use a library like graphql-fields. This helps you to extracts fields from the info parameter of the resolver function. Something like this: (as mentioned in the docs)

  import graphqlFields from 'graphql-fields';

  @Query(() => [Folder])
  folders(@Info() info: GraphQLResolveInfo): Promise<Folder[]> {
    // get fields
    const topLevelFields = Object.keys(graphqlFields(info));
    // query db and return results
  }

Now you can run your query based on the requested fields.