How to know which fields were requested in a GraphQL query?

I have written a GraphQL query which like the one below:

{
  posts {
    author {
      comments
    }
    comments
  }
}

I want to know how can I get the details about the requested child fields inside the posts resolver.

I want to do it to avoid nested calls of resolvers. I am using ApolloServer's DataSource API.

I can change the API server to get all the data at once.

I am using ApolloServer 2.0 and any other ways of avoiding nested calls are also welcome.

You'll need to parse the info object that's passed to the resolver as its fourth parameter. This is the type for the object:

type GraphQLResolveInfo = {
  fieldName: string,
  fieldNodes: Array<Field>,
  returnType: GraphQLOutputType,
  parentType: GraphQLCompositeType,
  schema: GraphQLSchema,
  fragments: { [fragmentName: string]: FragmentDefinition },
  rootValue: any,
  operation: OperationDefinition,
  variableValues: { [variableName: string]: any },
}

You could transverse the AST of the field yourself, but you're probably better off using an existing library. I'd recommend graphql-parse-resolve-info. There's a number of other libraries out there, but graphql-parse-resolve-info is a pretty complete solution and is actually used under the hood by postgraphile. Example usage:

posts: (parent, args, context, info) => {
  const parsedResolveInfo = parseResolveInfo(info)
  console.log(parsedResolveInfo)
}

This will log an object along these lines:

{
  alias: 'posts',
  name: 'posts',
  args: {},
  fieldsByTypeName: {
    Post: {
      author: {
        alias: 'author',
        name: 'author',
        args: {},
        fieldsByTypeName: ...
      }
      comments: {
        alias: 'comments',
        name: 'comments',
        args: {},
        fieldsByTypeName: ...
      }
    }
  }
}

You can walk through the resulting object and construct your SQL query (or set of API requests, or whatever) accordingly.