Resolver chains in GraphQL - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

Output of nested resolver chain

Given a GraphQL schema where a Book type has an author field resolved by a chain of resolvers, what is the output of this query?

{ book(id: 1) { title author { name } } }

Assume the resolvers are:
- book returns { id: 1, title: "1984", authorId: 42 }
- author resolver fetches author by authorId and returns { id: 42, name: "George Orwell" }

A {"data":{"book":{"title":"1984","authorId":42}}}

B {"data":{"book":{"title":"1984","author":{"name":"George Orwell"}}}}

C {"data":{"book":{"title":"1984","author":null}}}

D {"errors":[{"message":"Cannot resolve author"}]}

💡 Hint

Think about how nested resolvers return the final nested object.

The book resolver returns the book object with authorId. The author resolver uses that authorId to fetch and return the author object with the name. So the nested author field resolves to the author object with the name.

🧠 Conceptual

intermediate

Understanding resolver chain execution order

In a GraphQL query with nested fields, in what order are resolvers executed?

A Parent field resolvers execute first, then child field resolvers execute after parent resolves.

B All resolvers execute in parallel regardless of nesting.

C Child field resolvers execute first, then parent field resolvers.

D Resolvers execute randomly without order.

💡 Hint

Think about how the parent resolver must provide data for the child resolver.

Resolvers execute top-down. The parent resolver must finish and return data before the child resolver can run because the child depends on the parent's result.

📝 Syntax

advanced

Identify the syntax error in resolver chaining

Which resolver function below will cause a syntax error when used in a resolver chain?

A author: (parent) => fetchAuthorById(parent.authorId);

B author: async (parent) => { return await fetchAuthorById(parent.authorId); }

C author: (parent) => { fetchAuthorById(parent.authorId) }

D author: (parent) => fetchAuthorById(parent.authorId)

💡 Hint

Look for misplaced punctuation like semicolons.

Option A includes a trailing semicolon after the arrow function expression, causing a syntax error when defining the resolver as a property value in an object literal.

🔧 Debug

advanced

Why does the nested resolver return null?

A nested resolver in a chain returns null unexpectedly. The parent resolver returns an object with authorId, but the child resolver returns null. What is the most likely cause?

A The schema does not define the nested field.

B The parent resolver returned null instead of an object.

C The GraphQL query is missing the nested field.

D The child resolver is not using the correct field name from the parent object.

💡 Hint

Check how the child resolver accesses data from the parent.

If the child resolver expects a field like authorId but the parent object uses a different field name or the child accesses a wrong property, it will fail to fetch the nested data and return null.

❓ optimization

expert

Optimizing resolver chains to reduce database calls

In a resolver chain where multiple books each resolve their authors, how can you optimize to reduce the number of database calls?

A Remove the author field from the schema to avoid calls.

B Fetch authors inside each book resolver separately without optimization.

C Batch author fetches by collecting all authorIds and querying once in a single resolver.

D Cache each author fetch individually without batching.

💡 Hint

Think about how to fetch multiple related records efficiently.

Batching author fetches by collecting all needed authorIds and querying once reduces the number of database calls and improves performance.