Inline fragments in GraphQL - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

What is the output of this GraphQL query using inline fragments?

Given this GraphQL schema with types Book and Magazine implementing Publication, and this query:

{
  publications {
    title
    ... on Book {
      author
    }
    ... on Magazine {
      issue
    }
  }
}

Assuming the data has one Book titled "GraphQL Guide" by "Alice" and one Magazine titled "Tech Monthly" with issue 42, what is the query result?

A [{"title": "GraphQL Guide", "issue": 42}, {"title": "Tech Monthly", "author": "Alice"}]

B [{"title": "GraphQL Guide"}, {"title": "Tech Monthly"}]

C [{"title": "GraphQL Guide", "author": "Alice"}, {"title": "Tech Monthly", "issue": 42}]

D [{"author": "Alice"}, {"issue": 42}]

💡 Hint

Inline fragments allow you to select fields specific to a type when querying an interface or union.

The query returns a list of publications. For each item, it includes the title field, and then uses inline fragments to include author only if the item is a Book, and issue only if the item is a Magazine. So the Book has author, the Magazine has issue.

🧠 Conceptual

intermediate

Why use inline fragments in GraphQL queries?

Choose the best reason why inline fragments are used in GraphQL queries.

A To paginate large lists of data efficiently.

B To sort query results on the server side.

C To define reusable fragments that can be used across multiple queries.

D To conditionally select fields based on the runtime type of an object implementing an interface or union.

💡 Hint

Think about how GraphQL handles interfaces and unions with different possible types.

Inline fragments let you specify fields that only apply to certain types when querying interfaces or unions. This lets you get type-specific data in one query.

📝 Syntax

advanced

Identify the syntax error in this GraphQL query using inline fragments

Which option contains a syntax error in the use of inline fragments?

{
  search(term: "GraphQL") {
    ... on Book {
      title
      author
    }
    ... Magazine {
      title
      issue
    }
  }
}

A The fragment on <code>Book</code> should be named with a fragment name.

B The fragment on <code>Magazine</code> is missing the keyword <code>on</code>.

C Inline fragments cannot be used inside queries.

D The <code>search</code> field cannot have inline fragments.

💡 Hint

Check the syntax for inline fragments carefully.

Inline fragments require the keyword on before the type name. The fragment on Magazine is missing on, causing a syntax error.

❓ optimization

advanced

How do inline fragments improve query efficiency in GraphQL?

Which option best explains how inline fragments can optimize GraphQL queries?

A They allow fetching only the fields relevant to the actual type, reducing unnecessary data transfer.

B They automatically cache query results on the client side.

C They enable batch loading of related objects in one request.

D They compress the query string to reduce network size.

💡 Hint

Think about how selecting fields conditionally affects the data returned.

Inline fragments let you request fields only for the types that have them, so you avoid fetching irrelevant fields and reduce data size.

🔧 Debug

expert

What error does this GraphQL query raise?

Given this query:

{
  items {
    ... on Product {
      id
      price
    }
    ... on Service {
      id
      duration
    }
    ... on Product {
      name
    }
  }
}

Assuming items returns a list of Product and Service types, what error will this query cause?

A No error; the query runs successfully.

B Syntax error due to missing fragment names.

C Runtime error because <code>duration</code> is not a field of <code>Service</code>.

D Duplicate fragment type error because <code>Product</code> inline fragment is repeated.

💡 Hint

Inline fragments on the same type are allowed; GraphQL merges their field selections.

GraphQL allows multiple inline fragments on the same type at the same level. Their field selections are merged (e.g., id, price, and name for Product), so no error occurs.

Practice

1. What is the main purpose of using inline fragments in a GraphQL query?

easy

A. To select fields conditionally based on the object type in a mixed list

B. To create new types in the schema

C. To update data on the server

D. To paginate query results

Solution

1. Step 1: Understand inline fragments usage

   Inline fragments allow querying different fields depending on the type of an object in a list or union.

2. Step 2: Identify the purpose in queries

   This lets you get specific fields for each type without separate queries.

3. Final Answer:

   To select fields conditionally based on the object type in a mixed list -> Option A

4. Quick Check:

   Inline fragments = conditional fields by type [OK]

Hint: Inline fragments pick fields by type inside mixed lists [OK]

Common Mistakes:

• Thinking inline fragments create new types
• Confusing inline fragments with mutations
• Using inline fragments for pagination

2. Which of the following is the correct syntax for an inline fragment in GraphQL?

easy

A. ... on { TypeName field1 field2 }

B. ... TypeName { field1 field2 }

C. ... TypeName on { field1 field2 }

D. ... on TypeName { field1 field2 }

Solution

1. Step 1: Recall inline fragment syntax

   The correct syntax uses three dots, then on, then the type name, followed by fields in braces.

2. Step 2: Match options to syntax

   ... on TypeName { field1 field2 } matches this exactly: ... on TypeName { field1 field2 }.

3. Final Answer:

   ... on TypeName { field1 field2 } -> Option D

4. Quick Check:

   Inline fragment syntax = '... on TypeName { fields }' [OK]

Hint: Remember: three dots + 'on' + type name + braces [OK]

Common Mistakes:

• Omitting the 'on' keyword
• Placing type name inside braces incorrectly
• Using wrong order of keywords

3. Given this GraphQL query snippet:

{
  searchItems {
    id
    ... on Book {
      author
    }
    ... on Movie {
      director
    }
  }
}

What fields will be returned for an item of type Movie?

medium

A. id and director

B. id and author

C. only director

D. only id

Solution

1. Step 1: Identify fields outside and inside fragments

   The field id is always returned for all items. For Movie type, the inline fragment selects director.

2. Step 2: Combine fields for Movie type

   So, the result for a Movie item includes id and director.

3. Final Answer:

   id and director -> Option A

4. Quick Check:

   Movie fields = id + director [OK]

Hint: Fields outside fragments always return; inside depends on type [OK]

Common Mistakes:

• Assuming author field returns for Movie
• Ignoring fields outside fragments
• Thinking only fragment fields return

4. Consider this GraphQL query:

{
  items {
    ... on Book {
      title
      author
    }
    ... on Movie {
      title
      director
    }
  }
}

What is the error in this query?

medium

A. Cannot query both Book and Movie in the same query

B. Missing closing brace for the root field

C. Fields inside fragments must be unique across fragments

D. Inline fragments cannot be used without a type condition

Solution

1. Step 1: Check query braces

   The query opens braces for items and two inline fragments but does not close the items field or the root query braces.

2. Step 2: Identify missing closing braces

   Proper GraphQL syntax requires matching closing braces for all opened braces.

3. Final Answer:

   Missing closing brace for the root field -> Option B

4. Quick Check:

   All braces must close properly [OK]

Hint: Count opening and closing braces carefully [OK]

Common Mistakes:

• Forgetting to close braces
• Thinking inline fragments need unique fields
• Believing you cannot query multiple types

5. You have a GraphQL schema with a union type SearchResult = Book | Movie | Author. You want to query a list of searchResults and get the title for Book, director for Movie, and name for Author. Which query correctly uses inline fragments to achieve this?

hard

A. { searchResults { ... { title director name } } }

B. { searchResults { title director name } }

C. { searchResults { ... on Book { title } ... on Movie { director } ... on Author { name } } }

D. { searchResults { ... on SearchResult { title director name } } }

Solution

1. Step 1: Understand union type querying

   For a union, you must use inline fragments with ... on TypeName to get fields specific to each type.

2. Step 2: Evaluate each option

   { searchResults { ... on Book { title } ... on Movie { director } ... on Author { name } } } correctly uses inline fragments for each type. { searchResults { title director name } } tries to get all fields directly, which fails. { searchResults { ... { title director name } } } has invalid syntax. { searchResults { ... on SearchResult { title director name } } } incorrectly uses the union name as a type condition.

3. Final Answer:

   { searchResults { ... on Book { title } ... on Movie { director } ... on Author { name } } } -> Option C

4. Quick Check:

   Use inline fragments per type in unions [OK]

Hint: Use '... on TypeName' for each union member [OK]

Common Mistakes:

• Querying all fields without fragments on unions
• Using union name as type condition
• Incorrect fragment syntax