Interface types in GraphQL - Mini Project: Build & Apply

GraphQL Interface Types Project

📖 Scenario: You are building a simple GraphQL API for a library system. The library has different types of items: books and magazines. Both share some common fields like id and title, but also have their own unique fields.

🎯 Goal: Create a GraphQL schema using interface types to define common fields for library items, then implement two types Book and Magazine that implement this interface. Finally, create a query to fetch all library items.

📋 What You'll Learn

Define an interface called LibraryItem with fields id (ID!) and title (String!)

Create a type Book that implements LibraryItem and adds a author field (String!)

Create a type Magazine that implements LibraryItem and adds an issueNumber field (Int!)

Create a query libraryItems that returns a list of LibraryItem interface

💡 Why This Matters

🌍 Real World

Interfaces in GraphQL help model real-world entities that share common features but also have unique details, like different types of library items.

💼 Career

Understanding GraphQL interfaces is important for building flexible and scalable APIs that serve diverse data types with shared structure.

1

Define the LibraryItem interface

Create a GraphQL interface called LibraryItem with two fields: id of type ID! and title of type String!.

GraphQL

# Define the LibraryItem interface with id and title fields

Need a hint?

Use the interface keyword followed by the interface name and define the fields inside curly braces.

2

Create the Book and Magazine types implementing LibraryItem

Create a type called Book that implements LibraryItem and adds a field author of type String!. Also create a type called Magazine that implements LibraryItem and adds a field issueNumber of type Int!.

GraphQL

interface LibraryItem {
  id: ID!
  title: String!
}

# Define Book and Magazine types implementing LibraryItem

Need a hint?

Use the type keyword with implements LibraryItem and add the extra fields inside the type.

3

Add the libraryItems query returning a list of LibraryItem

Add a Query type with a field called libraryItems that returns a list of LibraryItem interface (use square brackets and exclamation mark for non-null list).

GraphQL

interface LibraryItem {
  id: ID!
  title: String!
}

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

type Magazine implements LibraryItem {
  id: ID!
  title: String!
  issueNumber: Int!
}

# Define Query type with libraryItems field returning [LibraryItem!]!

Need a hint?

Define a Query type with a field returning a non-null list of non-null LibraryItem.

4

Complete the schema with a union resolver placeholder

Add a placeholder comment inside the schema for resolvers to handle the LibraryItem interface type resolution (e.g., __resolveType). This completes the schema structure for interface usage.

GraphQL

interface LibraryItem {
  id: ID!
  title: String!
}

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

type Magazine implements LibraryItem {
  id: ID!
  title: String!
  issueNumber: Int!
}

type Query {
  libraryItems: [LibraryItem!]!
}

# Add a comment placeholder for interface resolver

Need a hint?

Resolvers for interfaces need a __resolveType function to tell GraphQL which type to use.