Why Union types in GraphQL? - Purpose & Use Cases

The Big Idea

What if you could handle many different data types as easily as one, without messy code?

The Scenario

Imagine you have a list of different shapes like circles, squares, and triangles, and you want to store their details together. Without union types, you might try to create separate lists or force all shapes into one format, which gets messy fast.

The Problem

Manually handling each shape type separately means writing lots of repetitive code and constantly checking which shape you are dealing with. This is slow, confusing, and easy to make mistakes, especially when adding new shapes.

The Solution

Union types let you group different types under one umbrella cleanly. You can say, 'This can be a circle or a square or a triangle,' and GraphQL will handle the rest. This keeps your code simple and flexible.

Before vs After

✗ Before

type Circle { radius: Float }
type Square { side: Float }
type ShapeList { circles: [Circle], squares: [Square] }

✓ After

union Shape = Circle | Square
type ShapeList { shapes: [Shape] }

What It Enables

Union types enable you to work with multiple related types seamlessly in one place, making your data queries simpler and more powerful.

Real Life Example

Think of a social media app where a feed can show posts, photos, or videos. Using union types, the feed can return any of these content types without extra hassle.

Key Takeaways

Union types group different types under one name.

They reduce repetitive code and errors.

They make queries flexible and easier to manage.

Practice

1. What is the main purpose of using union types in GraphQL?

easy

A. To group multiple object types into one field that can return different types

B. To define a list of scalar values

C. To create a new scalar type

D. To enforce a single object type for a field

Solution

1. Step 1: Understand union type purpose

   Union types allow a field to return one of several object types, grouping them logically.

2. Step 2: Compare with other options

   Defining a list of scalar values or creating a new scalar type describes scalars, not unions. Enforcing a single object type for a field contradicts the union concept.

3. Final Answer:

   To group multiple object types into one field that can return different types -> Option A

4. Quick Check:

   Union types = group multiple object types [OK]

Hint: Unions group different object types under one field [OK]

Common Mistakes:

• Confusing union with scalar types
• Thinking unions enforce a single type
• Mixing unions with interfaces

2. Which of the following is the correct syntax to define a union type named SearchResult that includes User and Post types?

easy

A. type SearchResult = User & Post

B. union SearchResult = User | Post

C. interface SearchResult = User | Post

D. union SearchResult { User, Post }

Solution

1. Step 1: Recall union syntax

   Unions use the syntax: union Name = Type1 | Type2 with pipe separators.

2. Step 2: Check each option

   union SearchResult = User | Post matches correct syntax. type SearchResult = User & Post uses & which is for intersections, not unions. interface SearchResult = User | Post wrongly uses interface keyword. union SearchResult { User, Post } uses braces which is invalid for unions.

3. Final Answer:

   union SearchResult = User | Post -> Option B

4. Quick Check:

   Union syntax uses '=' and '|' [OK]

Hint: Use '=' and '|' to define unions, no braces [OK]

Common Mistakes:

• Using '&' instead of '|'
• Using braces {} instead of '='
• Confusing union with interface syntax

3. Given the union type SearchResult = User | Post and this query:

{ search { ... on User { name } ... on Post { title } } }

What fields will be returned if the search result contains one User with name "Alice" and one Post with title "GraphQL Guide"?

medium

A. [{"name": "Alice"}]

B. [{"name": "Alice", "title": "GraphQL Guide"}]

C. [{"title": "GraphQL Guide"}]

D. [{"name": "Alice"}, {"title": "GraphQL Guide"}]

Solution

1. Step 1: Understand inline fragments on union

   The query uses inline fragments to select name from User and title from Post.

2. Step 2: Apply to data

   Since the result has one User and one Post, the response includes both objects separately with their respective fields.

3. Final Answer:

   [{"name": "Alice"}, {"title": "GraphQL Guide"}] -> Option D

4. Quick Check:

   Inline fragments return fields per type separately [OK]

Hint: Inline fragments return separate objects per type [OK]

Common Mistakes:

• Combining fields into one object
• Returning only one type's fields
• Ignoring inline fragment usage

4. Consider this union definition:

union SearchResult = User | Post

And this query:

{ search { ... on User { id name } ... on Post { id title } } }

Which of the following errors will occur if you try to query a field email inside Post inline fragment like this:

{ search { ... on User { id name } ... on Post { id title email } } }

medium

A. Error: Field 'email' must be queried on User type

B. No error, query runs successfully

C. Error: Field 'email' does not exist on type 'Post'

D. Error: Union types cannot have inline fragments

Solution

1. Step 1: Check Post type fields

   If email is not defined on Post type, querying it causes an error.

2. Step 2: Understand inline fragment validation

   Inline fragments must only query fields existing on the specified type. Querying unknown fields causes errors.

3. Final Answer:

   Error: Field 'email' does not exist on type 'Post' -> Option C

4. Quick Check:

   Querying unknown fields on type causes error [OK]

Hint: Check if field exists on type before querying [OK]

Common Mistakes:

• Assuming all fields exist on all union types
• Thinking unions disallow inline fragments
• Querying fields on wrong types

5. You have a union type SearchResult = User | Post | Comment. You want to write a query that returns the name for User, title for Post, and content for Comment. Which query correctly fetches these fields?

hard

A. { search { ... on User { name } ... on Post { title } ... on Comment { content } } }

B. { search { name title content } }

C. { search { ... on User { name } ... on Post { title } content } }

D. { search { ... on User { name } ... on Post { title } ... on Comment { title } } }

Solution

1. Step 1: Use inline fragments for each union type

   Each type in the union requires its own inline fragment to query its specific fields.

2. Step 2: Validate fields per type

   { search { ... on User { name } ... on Post { title } ... on Comment { content } } } queries name on User, title on Post, and content on Comment correctly. Other options either query fields directly without fragments or use wrong fields.

3. Final Answer:

   { search { ... on User { name } ... on Post { title } ... on Comment { content } } } -> Option A

4. Quick Check:

   Use inline fragments per type to query union fields [OK]

Hint: Use one inline fragment per union type with correct fields [OK]

Common Mistakes:

• Querying all fields directly without fragments
• Using wrong fields for a type
• Mixing fields inside fragments incorrectly