Inline fragments in GraphQL - Step-by-Step Execution

Concept Flow - Inline fragments

Start Query

↓

Check Type Condition?

↓

Apply Inline Fragment

↓

Collect Fields

↓

Return Result

The query starts and checks if the current data matches the inline fragment's type condition. If yes, it applies the fragment fields; if no, it skips them, then collects all fields and returns the result.

Execution Sample

GraphQL

query {
  search(text: "apple") {
    ... on Fruit {
      color
    }
    ... on Vegetable {
      size
    }
  }
}

This query searches for items named 'apple' and returns 'color' if the item is a Fruit, or 'size' if it is a Vegetable, using inline fragments.

Execution Table

Step	Current Item Type	Check Inline Fragment Type	Match?	Action	Fields Collected
1	Fruit	Fruit	Yes	Apply fragment	color
2	Fruit	Vegetable	No	Skip fragment	color
3	Vegetable	Fruit	No	Skip fragment	[]
4	Vegetable	Vegetable	Yes	Apply fragment	size
5	Grain	Fruit	No	Skip fragment
6	Grain	Vegetable	No	Skip fragment
7	-	-	-	Return collected fields	color or size or empty

💡 All inline fragments checked for each item type; fields collected accordingly; query execution ends.

Variable Tracker

Variable	Start	After 1	After 2	After 3	After 4	Final
Current Item Type	None	Fruit	Fruit	Vegetable	Vegetable	Grain
Fields Collected	[]	[color]	[color]	[]	[size]	[]

Key Moments - 2 Insights

Why does the query skip some inline fragments for certain item types?

Because inline fragments only apply if the current item's type matches the fragment's type condition, as shown in execution_table rows where 'Match?' is 'No' and action is 'Skip fragment'.

What happens if the item type does not match any inline fragment?

No fields from inline fragments are collected for that item, resulting in empty fields, as seen in execution_table rows 5 and 6.

Visual Quiz - 3 Questions

Look at the execution_table, what fields are collected when the current item type is 'Fruit'?

A size

B color

C color and size

D none

💡 Hint

Check rows 1 and 2 in execution_table where Current Item Type is 'Fruit' and see which fragment matched.

At which step does the inline fragment for 'Vegetable' get applied for a 'Vegetable' item?

A Step 4

B Step 3

C Step 2

D Step 5

💡 Hint

Look at execution_table row 4 where Current Item Type is 'Vegetable' and fragment type is 'Vegetable'.

If a new item type 'Berry' is added without a matching inline fragment, what will happen to fields collected?

A Fields from Fruit fragment will be collected

B Fields from Vegetable fragment will be collected

C No fields will be collected

D All fragment fields will be collected

💡 Hint

Refer to execution_table rows 5 and 6 where no fragment matches result in empty fields.

Concept Snapshot

Inline fragments let you conditionally select fields based on the type of the data.
Syntax: ... on TypeName { fields }
If the data matches TypeName, fields inside the fragment are included.
If not, those fields are skipped.
Useful for querying interfaces or unions with different types.

Full Transcript

Inline fragments in GraphQL allow you to ask for fields only if the data is of a certain type. When the query runs, it checks each item’s type against the inline fragment’s type condition. If they match, it collects the fields inside that fragment. If not, it skips them. This way, you can write one query that works for different types of data, like Fruits and Vegetables, and get the right fields for each. The execution table shows how for each item type, the query checks fragments and collects fields accordingly. If no fragment matches, no fields from fragments are collected. This helps you get exactly the data you want depending on the 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