Nested resolver execution in GraphQL - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

What is the output of nested resolvers in this GraphQL query?

Given the following GraphQL schema and resolvers, what will be the output of the query?

type Query {
  user(id: ID!): User
}

type User {
  id: ID!
  name: String!
  posts: [Post!]!
}

type Post {
  id: ID!
  title: String!
  comments: [Comment!]!
}

type Comment {
  id: ID!
  content: String!
}

Query:
{
  user(id: "1") {
    name
    posts {
      title
      comments {
        content
      }
    }
  }
}

Resolvers return these data objects:

user: { id: "1", name: "Alice", posts: [{ id: "101", title: "Hello World", comments: [{ id: "1001", content: "Nice post!" }] }] }

A {"data":{"user":{"name":"Alice","posts":[{"title":"Hello World","comments":[{"content":"Nice post!"}]}]}}}

B {"data":{"user":{"name":"Alice","posts":[{"title":"Hello World","comments":[]}]}}}

C {"data":{"user":{"name":"Alice","posts":[]}}}

D {"data":{"user":null}}

💡 Hint

Think about how nested resolvers return data objects and how GraphQL assembles the response.

The nested resolvers for posts and comments return the full data objects as specified. The query asks for name, posts.title, and comments.content. The output JSON reflects this nested structure with all requested fields.

🧠 Conceptual

intermediate

How does GraphQL execute nested resolvers?

When a GraphQL query requests nested fields, how does the execution of resolvers proceed?

A GraphQL executes all resolvers in parallel regardless of nesting order.

B GraphQL executes nested resolvers before parent resolvers.

C GraphQL executes only the top-level resolvers and ignores nested resolvers.

D GraphQL executes parent resolvers first, then executes child resolvers for nested fields sequentially.

💡 Hint

Think about how data dependencies flow from parent to child in nested queries.

GraphQL first resolves the parent field to get its data, then uses that data to resolve nested fields. This ensures nested resolvers have the context they need.

📝 Syntax

advanced

Identify the syntax error in this nested resolver function

Consider this JavaScript resolver function for a nested field in GraphQL:

const resolvers = {
  User: {
    posts(parent, args, context) {
      return context.db.posts.filter(post => post.userId === parent.id)
    },
    comments(parent, args, context) {
      return context.db.comments.filter(comment => comment.postId === parent.id)
    }
  }
}

What is the syntax error here?

A Missing comma between 'posts' and 'comments' resolver functions.

B Incorrect use of 'parent' parameter in 'comments' resolver.

C Missing return statement in 'posts' resolver.

D Using 'filter' method on undefined 'context.db.posts'.

💡 Hint

Check the object literal syntax inside the resolver map.

In JavaScript object literals, properties must be separated by commas. Here, the 'posts' and 'comments' resolver functions lack a comma between them, causing a syntax error.

🔧 Debug

advanced

Why does this nested resolver return empty arrays?

Given this resolver snippet:

const resolvers = {
  User: {
    posts(parent, args, context) {
      return context.db.posts.filter(post => post.userId === parent.id)
    }
  },
  Post: {
    comments(parent, args, context) {
      return context.db.comments.filter(comment => comment.postId === parent.id)
    }
  }
}

And the query:

{ user(id: "1") { posts { title comments { content } } } }

The response shows posts with titles but empty comments arrays. What is the most likely cause?

A The 'comments' resolver uses 'parent.id' but should use 'parent.postId'.

B The 'posts' resolver returns empty array, so no comments are fetched.

C The 'comments' resolver is not nested under 'Post' type in the schema.

D The 'comments' resolver is missing a return statement.

💡 Hint

Check if the resolver matches the schema type it belongs to.

If the 'comments' resolver is not correctly assigned to the 'Post' type in the schema, GraphQL will not call it for nested 'comments' fields, resulting in empty arrays.

❓ optimization

expert

How to optimize nested resolver execution to reduce database calls?

In a GraphQL API, nested resolvers for 'posts' and 'comments' cause many database calls, slowing response time. Which approach best optimizes this?

A Use multiple separate database connections for each nested resolver.

B Use DataLoader to batch and cache database requests for nested fields.

C Remove nested resolvers and fetch all data in the root resolver only.

D Cache the entire GraphQL response on the client side.

💡 Hint

Think about batching and caching repeated database queries during nested resolution.

DataLoader batches multiple requests for the same data into a single database call and caches results, reducing redundant queries in nested resolvers.