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

Question

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.

Answer 1

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.