Migration from REST to GraphQL - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

Query Result: Fetching Nested Data in GraphQL

You have a REST API endpoint /users/1 that returns user data with their posts. In GraphQL, you want to fetch the same data with one query.

Given this GraphQL query:

{ user(id: 1) { id name posts { id title } } }

What is the expected JSON output?

A {"user":{"id":1,"name":"Alice","posts":[{"id":101,"title":"Hello World"},{"id":102,"title":"GraphQL Rocks"}]}}

B {"data":{"user":{"id":1,"name":"Alice","posts":[{"id":101,"title":"Hello World"},{"id":102,"title":"GraphQL Rocks"}]}}}

C {"data":{"user":{"id":1,"name":"Alice"}}}

D {"data":{"users":[{"id":1,"name":"Alice","posts":[{"id":101,"title":"Hello World"}]}]}}

💡 Hint

GraphQL responses always wrap the result inside a data field matching the query shape.

The GraphQL response wraps the requested fields inside a data object. The query asks for user with id 1, their id, name, and posts with id and title. Option B matches this exactly.

🧠 Conceptual

intermediate

Understanding GraphQL Query vs REST Endpoint

Which statement best describes the difference between a GraphQL query and a REST API endpoint?

A A GraphQL query allows clients to specify exactly what data they want, while a REST endpoint returns a fixed data structure.

B A REST endpoint can return nested data in one call, but GraphQL requires multiple queries.

C GraphQL queries always require multiple network requests, unlike REST endpoints.

D REST APIs do not support filtering data, but GraphQL does not support filtering either.

💡 Hint

Think about flexibility in data retrieval.

GraphQL lets clients ask for exactly the fields they want, reducing over-fetching. REST endpoints return a fixed response shape, often requiring multiple calls for nested data.

📝 Syntax

advanced

Identify the Syntax Error in GraphQL Mutation

Consider this GraphQL mutation intended to create a new user:

mutation {
  createUser(name: "Bob", age: 30) {
    id
    name
  }
}

Which option shows the correct syntax to fix the error?

A

mutation {
  createUser(name: "Bob" age: 30) {
    id
    name
  }
}

B

mutation {
  createUser(name: "Bob", age: 30) {
    id
    name
  }
}

C

mutation {
  createUser(input: {name: "Bob", age: 30}) {
    id
    name
  }
}

D

mutation {
  createUser(name: "Bob", age: "30") {
    id
    name
  }
}

💡 Hint

Check how arguments are passed in mutations.

Most GraphQL mutations expect a single input object argument. Option C correctly wraps the fields inside an input object. Option C is invalid if the schema requires an input object. Option C is missing a comma. Option C passes age as a string instead of an integer.

❓ optimization

advanced

Optimizing GraphQL Queries to Reduce Overfetching

You have a REST endpoint /products returning all product details. Migrating to GraphQL, you want to optimize queries to fetch only product names and prices.

Which GraphQL query best achieves this?

A { products { name price } }

B { products { id name price description stock } }

C { products { name price stock } }

D { products { id name } }

💡 Hint

Only request the fields you need.

Option A requests only name and price, minimizing data transfer and processing. Other options request extra fields, causing overfetching.

🔧 Debug

expert

Debugging a GraphQL Schema Migration Issue

After migrating from REST to GraphQL, a query to fetch a user's email returns null even though the REST API returned the email correctly.

Given this GraphQL schema snippet:

type User {
  id: ID!
  name: String!
  email: String
}

And resolver code snippet:

user: (parent, args, context) => {
  return database.findUserById(args.id);
}

What is the most likely cause of the null email in the response?

A The client query does not request the email field.

B The GraphQL schema incorrectly marks email as nullable, causing it to be null.

C The resolver function is missing a return statement.

D The database query does not select the email field, so it is missing in the returned user object.

💡 Hint

Check what data the resolver returns from the database.

If the database query does not include the email field, the resolver returns a user object without email, so GraphQL returns null for that field. The schema marking email as nullable is correct to allow nulls if missing. The resolver has a return statement. If the client did not request email, it wouldn't appear at all.