Inline fragments in GraphQL

Introduction

Inline fragments let you ask for different fields depending on the type of data you get back. This helps when you have mixed types in one query.

When you query a list that contains different types of items and want specific fields for each type.

When you want to avoid multiple queries by getting all needed data in one request.

When your data has optional fields that only exist on some types.

When you want to keep your query simple but still get detailed info for different types.

Syntax

GraphQL

query {
  someField {
    ... on TypeName {
      field1
      field2
    }
    ... on AnotherType {
      fieldA
      fieldB
    }
  }
}

Use ... on TypeName inside your query to specify fields for that type.

Inline fragments help when the server returns different types in the same field.

Examples

This query asks for name and email if the item is a User, or title and price if it is a Product.

GraphQL

query {
  searchResults {
    ... on User {
      name
      email
    }
    ... on Product {
      title
      price
    }
  }
}

This query gets a common message for all notifications, but also gets severity if it is an Alert, or remindAt if it is a Reminder.

GraphQL

query {
  notifications {
    message
    ... on Alert {
      severity
    }
    ... on Reminder {
      remindAt
    }
  }
}

Sample Program

This query fetches a list of items. For each item, it always gets the id. If the item is a Book, it also gets the title and author. If it is a Movie, it gets the title and director.

GraphQL

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

Important Notes

Inline fragments only work when the server schema supports multiple types for a field.

You can mix inline fragments with common fields outside them.

Summary

Inline fragments let you get different fields based on the data type.

They help when querying mixed-type lists or unions.

Use ... on TypeName inside your query to specify fields for each type.

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