Schema testing in GraphQL - Mini Project: Build & Apply

GraphQL Schema Testing Basics

📖 Scenario: You are building a simple GraphQL API for a bookstore. You want to make sure your schema is correct before adding real data.

🎯 Goal: Create a GraphQL schema with a Book type and a Query type. Then write a test query to check the schema structure.

📋 What You'll Learn

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

Define a Query type with a field books that returns a list of Book

Write a test query to fetch id and title of all books

Ensure the schema is syntactically valid and the test query matches the schema

💡 Why This Matters

🌍 Real World

GraphQL schemas define how clients can request data from APIs in a clear and structured way.

💼 Career

Understanding schema design and testing is essential for backend developers working with GraphQL APIs.

1

Define the Book type

Create a GraphQL type called Book with these exact fields: id of type ID!, title of type String!, and author of type String!.

GraphQL

# Define the Book type
# Your code here

Hint

Use the type keyword followed by the type name and curly braces to define fields.

2

Add the Query type

Add a GraphQL Query type with a field called books that returns a list of Book (use square brackets).

GraphQL

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

# Define the Query type with books field
# Your code here

Hint

The Query type defines entry points for fetching data. Use square brackets to indicate a list.

3

Write a test query

Write a GraphQL query named GetBooks that fetches the id and title fields from the books query.

GraphQL

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

type Query {
  books: [Book]
}

# Write the GetBooks query
# Your code here

Hint

Use the query keyword followed by the query name and select the fields inside curly braces.

4

Complete the schema with a simple resolver placeholder

Add a placeholder resolver function for books that returns an empty array. Use JavaScript syntax for the resolver object with a Query field containing books as a function returning [].

GraphQL

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

type Query {
  books: [Book]
}

query GetBooks {
  books {
    id
    title
  }
}

// Add resolver placeholder
// Your code here

Hint

Resolvers connect schema fields to data. Here, return an empty array to keep it simple.

Practice

1. What is the main purpose of schema testing in GraphQL?

easy

A. To check the database connection

B. To test the speed of GraphQL queries

C. To verify that the GraphQL schema matches the expected structure and types

D. To validate user authentication tokens

Solution

1. Step 1: Understand schema testing purpose

   Schema testing ensures the GraphQL schema is correct and matches the design.

2. Step 2: Compare options to purpose

   Only verifying schema structure and types matches schema testing's goal.

3. Final Answer:

   To verify that the GraphQL schema matches the expected structure and types -> Option C

4. Quick Check:

   Schema testing = verify schema structure [OK]

Hint: Schema testing checks schema structure, not performance or auth [OK]

Common Mistakes:

• Confusing schema testing with performance testing
• Thinking schema testing checks database connections
• Assuming schema testing validates user authentication

2. Which of the following is the correct syntax to define a GraphQL schema type for a User with fields id (ID!) and name (String)?

easy

A. User { id: ID! name: String }

B. schema User { id: ID! name: String }

C. type User (id: ID!, name: String)

D. type User { id: ID! name: String }

Solution

1. Step 1: Recall GraphQL type syntax

   GraphQL types use the keyword type followed by name and curly braces with fields.

2. Step 2: Match syntax to options

   type User { id: ID! name: String } correctly uses type User { id: ID! name: String }. Others misuse keywords or punctuation.

3. Final Answer:

   type User { id: ID! name: String } -> Option D

4. Quick Check:

   Correct type syntax = type User { id: ID! name: String } [OK]

Hint: GraphQL types start with 'type' keyword and use braces {} [OK]

Common Mistakes:

• Using 'schema' instead of 'type' keyword
• Using parentheses instead of braces
• Omitting the 'type' keyword

3. Given this GraphQL schema snippet:

type Query { user(id: ID!): User }

And this query:

{ user(id: "123") { id name } }

What is the expected shape of the response data?

medium

A. {"user": {"id": "123", "name": "Alice"}}

B. {"data": {"user": {"id": "123", "name": "Alice"}}}

C. {"data": {"user": null}}

D. {"error": "User not found"}

Solution

1. Step 1: Understand GraphQL response format

   GraphQL responses wrap results inside a data object with requested fields.

2. Step 2: Match query and schema to response

   The query requests user with id "123" and fields id and name. Assuming user exists, response includes these inside data.

3. Final Answer:

   {"data": {"user": {"id": "123", "name": "Alice"}}} -> Option B

4. Quick Check:

   GraphQL response wraps data in 'data' key [OK]

Hint: GraphQL responses always wrap results inside 'data' key [OK]

Common Mistakes:

• Returning data without 'data' wrapper
• Confusing null user with error response
• Assuming error is returned instead of null

4. You wrote this schema test to check if the email field exists on User type:

expect(schema.getType('User').getFields()).toHaveProperty('email')

But the test fails. What is the most likely cause?

medium

A. The User type does not have an email field defined

B. The getFields() method is not valid on schema types

C. The test syntax is incorrect and should use hasProperty

D. The schema variable is undefined

Solution

1. Step 1: Understand what getFields() returns

   The getFields() method returns an object of fields defined on the type.

2. Step 2: Analyze test failure reason

   If test fails checking for 'email', likely the User type lacks that field in schema definition.

3. Final Answer:

   The User type does not have an email field defined -> Option A

4. Quick Check:

   Missing field causes test failure [OK]

Hint: Test fails if field is missing in schema type [OK]

Common Mistakes:

• Assuming method getFields() is invalid
• Confusing test assertion method names
• Not checking if schema variable is defined

5. You want to write a schema test to ensure the Post type has a field comments that returns a list of Comment types. Which test code correctly verifies this?

hard

A. expect(schema.getType('Post').getFields().comments.type.toString()).toBe('[Comment!]!')

B. expect(schema.getType('Post').getFields().comments.type.ofType.name).toBe('Comment')

C. expect(schema.getType('Post').getFields().comments.type.toString()).toBe('[Comment]')

D. expect(schema.getType('Post').getFields().comments.type.name).toBe('Comment')

Solution

1. Step 1: Understand GraphQL list and non-null syntax

   A list of Comment types with non-null items and non-null list is represented as [Comment!]!.

2. Step 2: Match test code to expected type string

   expect(schema.getType('Post').getFields().comments.type.toString()).toBe('[Comment!]!') checks the full type string including non-null markers, correctly verifying the list of non-null Comments.

3. Final Answer:

   expect(schema.getType('Post').getFields().comments.type.toString()).toBe('[Comment!]!') -> Option A

4. Quick Check:

   List of non-null Comments = '[Comment!]!' [OK]

Hint: Use toString() to check full list and non-null type syntax [OK]

Common Mistakes:

• Checking only inner type name without list brackets
• Ignoring non-null markers in type string
• Using wrong property to access type name