Nested resolver execution in GraphQL - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to define a nested resolver for the 'author' field inside 'Book' type.

GraphQL

const resolvers = {
  Book: {
    author: (parent, args, context, info) => [1]
  }
};

Drag options to blanks, or click blank then click option'

Aargs.id

Bparent.authorId

Ccontext.db.getAuthorById(parent.authorId)

Dinfo.fieldName

Attempts:

3 left

💡 Hint

Common Mistakes

Returning 'parent.authorId' directly without fetching author data.

Using 'args.id' which is not provided in this context.

The resolver for 'author' should fetch the author data using the 'authorId' from the parent 'Book' object, typically via a database call in context.

2fill in blank

medium

Complete the resolver to fetch nested comments for a 'Post' type.

GraphQL

const resolvers = {
  Post: {
    comments: (parent, args, context) => [1]
  }
};

Drag options to blanks, or click blank then click option'

Acontext.db.getPostById(args.id)

Bparent.comments

Cargs.postId

Dcontext.db.getCommentsByPostId(parent.id)

Attempts:

3 left

💡 Hint

Common Mistakes

Returning 'parent.comments' assuming comments are already loaded.

Using 'args.postId' which may not be provided.

The 'comments' resolver should fetch comments related to the post using the post's 'id' from the parent object.

3fill in blank

hard

Fix the error in the nested resolver to correctly fetch the 'profile' for a 'User'.

GraphQL

const resolvers = {
  User: {
    profile: (parent, args, context) => [1]
  }
};

Drag options to blanks, or click blank then click option'

Acontext.db.getProfileByUserId(parent.id)

Bcontext.db.getProfileByUserId(args.id)

Cparent.profile

Dcontext.db.getUserById(parent.profileId)

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'args.id' which may not be passed to this resolver.

Returning 'parent.profile' assuming profile is already loaded.

The resolver should use the user's 'id' from the parent to fetch the profile. Using 'args.id' or 'parent.profile' is incorrect here.

4fill in blank

hard

Fill both blanks to correctly resolve nested 'orders' and their 'items' for a 'Customer'.

GraphQL

const resolvers = {
  Customer: {
    orders: (parent, args, context) => [1]
  },
  Order: {
    items: (parent, args, context) => [2]
  }
};

Drag options to blanks, or click blank then click option'

Acontext.db.getOrdersByCustomerId(parent.id)

Bcontext.db.getItemsByOrderId(parent.id)

Cparent.orders

Dparent.items

Attempts:

3 left

💡 Hint

Common Mistakes

Returning 'parent.orders' or 'parent.items' assuming data is preloaded.

Mixing up customer id and order id in the resolvers.

The 'orders' resolver fetches orders by customer id, and the 'items' resolver fetches items by order id from the parent object.

5fill in blank

hard

Fill all three blanks to correctly resolve nested 'categories', 'products', and 'reviews' in a GraphQL schema.

GraphQL

const resolvers = {
  Category: {
    products: (parent, args, context) => [1]
  },
  Product: {
    reviews: (parent, args, context) => [2]
  },
  Review: {
    author: (parent, args, context) => [3]
  }
};

Drag options to blanks, or click blank then click option'

Acontext.db.getProductsByCategoryId(parent.id)

Bcontext.db.getReviewsByProductId(parent.id)

Ccontext.db.getUserById(parent.authorId)

Dparent.products

Attempts:

3 left

💡 Hint

Common Mistakes

Returning parent fields directly without fetching related data.

Using wrong parent properties like 'parent.productId' in the wrong resolver.

Each resolver fetches related data using the parent's id or authorId. Categories fetch products, products fetch reviews, and reviews fetch their author.

Practice

1. What is the main purpose of nested resolver execution in GraphQL?

easy

A. To convert GraphQL queries into SQL statements

B. To execute multiple queries in parallel without relation

C. To cache all data before sending a response

D. To fetch related data step-by-step using parent data

Solution

1. Step 1: Understand nested resolver role

   Nested resolvers use parent data to fetch related child data step-by-step.

2. Step 2: Compare other options

   Options B, C, and D describe unrelated or incorrect behaviors for nested resolvers.

3. Final Answer:

   To fetch related data step-by-step using parent data -> Option D

4. Quick Check:

   Nested resolver execution = step-by-step data fetch [OK]

Hint: Nested resolvers use parent info to get related data [OK]

Common Mistakes:

• Thinking nested resolvers run unrelated queries
• Confusing caching with resolver execution
• Assuming nested resolvers convert queries to SQL

2. Which of the following is the correct syntax for a nested resolver function in GraphQL?

easy

A. resolver(parent.id, fetchChildData)

B. fetchChildData(parent.id) => parent

C. parent => fetchChildData(parent.id)

D. child => parent.fetchData()

Solution

1. Step 1: Identify correct resolver signature

   Nested resolvers receive the parent object and use its fields to fetch related data, e.g., parent => fetchChildData(parent.id).

2. Step 2: Eliminate incorrect syntax

   Options B, C, and D do not follow the standard resolver function pattern.

3. Final Answer:

   parent => fetchChildData(parent.id) -> Option C

4. Quick Check:

   Nested resolver syntax = parent => fetchChildData(parent.id) [OK]

Hint: Nested resolver gets parent, then fetches child data [OK]

Common Mistakes:

• Reversing function parameters
• Using child as input instead of parent
• Incorrect arrow function placement

3. Given this GraphQL resolver snippet:

const resolvers = {
  User: {
    posts: (parent) => {
      return posts.filter(post => post.userId === parent.id);
    }
  }
};

What will User.posts return when called with parent = { id: 2 } if posts = [{id:1,userId:2},{id:2,userId:3},{id:3,userId:2}]?

medium

A. Error: posts is undefined

B. [{id:1,userId:2},{id:3,userId:2}]

C. []

D. [{id:2,userId:3}]

Solution

1. Step 1: Filter posts by parent.id

   The resolver filters posts where post.userId equals parent.id (2), so posts with userId 2 are selected.

2. Step 2: Identify matching posts

   Posts with id 1 and 3 have userId 2, so they are returned.

3. Final Answer:

   [{id:1,userId:2},{id:3,userId:2}] -> Option B

4. Quick Check:

   Filter posts by userId=2 = matching posts [OK]

Hint: Filter posts where userId matches parent.id [OK]

Common Mistakes:

• Returning posts with wrong userId
• Confusing parent.id with post.id
• Assuming empty result without checking data

4. Consider this nested resolver code:

const resolvers = {
  Author: {
    books: (parent) => {
      return books.filter(book => book.authorId = parent.id);
    }
  }
};

What is the error in this resolver?

medium

A. Using assignment '=' instead of comparison '===' in filter

B. Missing return statement in resolver

C. Incorrect resolver name 'books' instead of 'bookList'

D. Parent argument is not passed to resolver

Solution

1. Step 1: Identify filter condition mistake

   The filter uses '=' which assigns value instead of '===' which compares values.

2. Step 2: Understand impact of '=' in filter

   Using '=' causes all books to be included or unexpected behavior, not filtering correctly.

3. Final Answer:

   Using assignment '=' instead of comparison '===' in filter -> Option A

4. Quick Check:

   Filter condition must use '===' not '=' [OK]

Hint: Use '===' for comparison inside filter functions [OK]

Common Mistakes:

• Confusing '=' with '===' in conditions
• Forgetting to return filtered array
• Assuming resolver name must match data variable

5. You have a GraphQL schema with Author and nested books resolver. How would you optimize nested resolver execution to avoid fetching all books for each author individually?

hard

A. Batch book fetching using DataLoader to group requests by authorId

B. Fetch all books once and filter in each resolver call

C. Remove nested resolver and fetch books separately in client

D. Use multiple nested resolvers for each book field

Solution

1. Step 1: Understand nested resolver inefficiency

   Fetching books individually per author causes many database calls, slowing performance.

2. Step 2: Apply batching with DataLoader

   DataLoader batches requests by authorId, fetching all needed books in one query efficiently.

3. Final Answer:

   Batch book fetching using DataLoader to group requests by authorId -> Option A

4. Quick Check:

   Batching nested fetches = DataLoader usage [OK]

Hint: Use DataLoader to batch nested resolver database calls [OK]

Common Mistakes:

• Fetching all books every time wastes memory
• Removing nested resolver loses data relation
• Using multiple nested resolvers increases calls