Union types in GraphQL - Mini Project: Build & Apply

GraphQL Union Types for Media Library

📖 Scenario: You are building a media library database that stores different types of media items: Books and Movies. Each media type has its own unique fields.To efficiently query these different media types in one request, you will use GraphQL union types.

🎯 Goal: Create a GraphQL schema that defines Book and Movie types, then create a union type called MediaItem that can represent either a Book or a Movie. Finally, define a query mediaLibrary that returns a list of MediaItem.

📋 What You'll Learn

Define a Book type with fields id (ID!), title (String!), and author (String!).

Define a Movie type with fields id (ID!), title (String!), and director (String!).

Create a union type called MediaItem that includes Book and Movie.

Define a Query type with a field mediaLibrary that returns a list of MediaItem.

💡 Why This Matters

🌍 Real World

Union types let you query different types of related data in one request, useful in media libraries, e-commerce, or social networks.

💼 Career

Understanding union types is important for building flexible GraphQL APIs that handle multiple data types efficiently.

1

Define the Book and Movie types

Create a GraphQL schema that defines a Book type with fields id of type ID!, title of type String!, and author of type String!. Also define a Movie type with fields id of type ID!, title of type String!, and director of type String!.

GraphQL

# Define Book and Movie types here

Hint

Use type keyword to define both Book and Movie types with their respective fields.

2

Create the MediaItem union type

Add a union type called MediaItem that includes Book and Movie.

GraphQL

type Book {
  id: ID!
  title: String!
  author: String!
}

type Movie {
  id: ID!
  title: String!
  director: String!
}

# Define the MediaItem union type here

Hint

Use the union keyword followed by the union name and the types separated by |.

3

Define the Query type with mediaLibrary field

Add a Query type with a field called mediaLibrary that returns a list of MediaItem (use square brackets).

GraphQL

type Book {
  id: ID!
  title: String!
  author: String!
}

type Movie {
  id: ID!
  title: String!
  director: String!
}

union MediaItem = Book | Movie

# Define Query type with mediaLibrary field here

Hint

Define a Query type with a field mediaLibrary that returns a list of MediaItem using square brackets.

4

Complete the schema with schema definition

Add the schema definition that sets the query root type to Query.

GraphQL

type Book {
  id: ID!
  title: String!
  author: String!
}

type Movie {
  id: ID!
  title: String!
  director: String!
}

union MediaItem = Book | Movie

type Query {
  mediaLibrary: [MediaItem]
}

# Add schema definition here

Hint

Use the schema keyword and set query: Query to complete the schema.

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