Fragments for reusable selections in GraphQL - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

What is the output of this GraphQL query using fragments?

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

fragment userFields on User {
  id
  name
}

query {
  users {
    ...userFields
    email
  }
}

GraphQL

fragment userFields on User {
  id
  name
}

query {
  users {
    ...userFields
    email
  }
}

A [{"id": "1", "name": "Alice", "email": "[e-mail]"}, {"id": "2", "name": "Bob", "email": "[e-mail]"}]

B [{"id": "1", "name": "Alice"}, {"id": "2", "name": "Bob"}]

C [{"email": "[e-mail]"}, {"email": "[e-mail]"}]

D SyntaxError: Fragment 'userFields' is not defined

💡 Hint

Fragments allow you to reuse fields. The query includes the fragment and adds the email field.

The fragment userFields selects id and name. The query also selects email. So each user object includes id, name, and email.

🧠 Conceptual

intermediate

Why use fragments in GraphQL queries?

What is the main benefit of using fragments in GraphQL queries?

A To speed up the server response time by caching fragments

B To automatically generate mutations from queries

C To encrypt sensitive data in the query

D To reuse sets of fields across multiple queries or parts of a query

💡 Hint

Think about how fragments help avoid repeating the same fields.

Fragments let you define a group of fields once and reuse them in many places, making queries easier to maintain and less error-prone.

📝 Syntax

advanced

Identify the syntax error in this fragment usage

Which option correctly fixes the syntax error in this GraphQL query?

fragment userFields on User {
  id
  name
}

query {
  users {
    ...userFields
    email
  }
  ...userFields
}

GraphQL

fragment userFields on User {
  id
  name
}

query {
  users {
    ...userFields
    email
  }
  ...userFields
}

A Remove the spread outside the users field; fragments can only be spread inside fields of matching type

B Change 'fragment userFields on User' to 'fragment userFields on Query'

C Add parentheses after ...userFields like ...userFields()

D Move the fragment definition inside the query block

💡 Hint

Fragments must be spread only inside fields of the correct type.

The spread ...userFields outside the users field is invalid because userFields applies to User type, not the root query type.

❓ optimization

advanced

How do fragments improve query performance?

Which statement best describes how using fragments can improve GraphQL query performance?

A Fragments automatically batch multiple queries into one request

B Fragments reduce the size of the query sent over the network by avoiding repeated fields

C Fragments enable the server to cache and reuse parsed query parts internally

D Fragments compress the response data to reduce bandwidth

💡 Hint

Think about how the server processes queries with fragments.

Fragments allow the server to parse and cache parts of the query, which can speed up query execution internally, though they do not reduce the query size sent over the network.

🔧 Debug

expert

Why does this fragment cause a runtime error?

Given this fragment and query, why does the query fail at runtime?

fragment postFields on Post {
  id
  title
  author {
    id
    name
  }
}

query {
  posts {
    ...postFields
    author {
      email
    }
  }
}

GraphQL

fragment postFields on Post {
  id
  title
  author {
    id
    name
  }
}

query {
  posts {
    ...postFields
    author {
      email
    }
  }
}

A The fragment 'postFields' is missing the 'email' field for author, causing a null pointer error

B The query tries to select 'author' fields twice with conflicting selections, causing a merge conflict error

C Fragments cannot contain nested fields, so 'author' inside the fragment is invalid

D The query is missing a required argument for 'posts', causing a validation error

💡 Hint

Consider how GraphQL merges fields with the same name but different selections.

The query selects author twice with different subfields (id, name in the fragment and email in the query). GraphQL requires these selections to be compatible, so this causes a conflict error.