Why mutations modify data in GraphQL - See It in Action

Understanding Why Mutations Modify Data in GraphQL

📖 Scenario: You are building a simple GraphQL API for a small library. The library keeps track of books and their availability. You want to learn how to change data using GraphQL mutations.

🎯 Goal: Build a GraphQL mutation that modifies the availability status of a book in the library database.

📋 What You'll Learn

Create a GraphQL type Book with fields id, title, and available.

Create a mutation called updateBookAvailability that takes id and available as inputs.

The mutation should update the available status of the book with the given id.

Return the updated Book object from the mutation.

💡 Why This Matters

🌍 Real World

GraphQL mutations are used in real applications to change data on the server, such as updating user profiles, posting comments, or changing inventory status.

💼 Career

Understanding how mutations modify data is essential for backend developers working with GraphQL APIs to build interactive and dynamic applications.

1

Define the Book type

Create a GraphQL type called Book with fields id of type ID!, title of type String!, and available of type Boolean!.

GraphQL

# Define the Book type with id, title, and available fields
# Your code here

Hint

Use the type keyword to define a new GraphQL type. Make sure to use exclamation marks ! to mark fields as required.

2

Add the updateBookAvailability mutation signature

Add a mutation called updateBookAvailability that takes two arguments: id of type ID! and available of type Boolean!. The mutation should return a Book type.

GraphQL

type Book {
  id: ID!
  title: String!
  available: Boolean!
}

# Add the mutation signature for updateBookAvailability
# Your code here

Hint

Define a Mutation type and add the mutation with the correct argument types and return type.

3

Implement the resolver logic to modify data

Write a resolver function called updateBookAvailability that takes id and available as arguments, finds the book by id in the books array, updates its available field, and returns the updated book.

GraphQL

type Book {
  id: ID!
  title: String!
  available: Boolean!
}

type Mutation {
  updateBookAvailability(id: ID!, available: Boolean!): Book
}

const books = [
  { id: "1", title: "1984", available: true },
  { id: "2", title: "Brave New World", available: true }
];

// Write the updateBookAvailability resolver function
// Your code here

Hint

Use Array.find() to locate the book by id. Then change its available property and return the updated book.

4

Complete the schema with Query and Mutation types

Add a Query type with a field books that returns a list of Book. Also, export the full schema including Query and Mutation types.

GraphQL

type Book {
  id: ID!
  title: String!
  available: Boolean!
}

type Mutation {
  updateBookAvailability(id: ID!, available: Boolean!): Book
}

type Query {
  # Add books field here
}

const books = [
  { id: "1", title: "1984", available: true },
  { id: "2", title: "Brave New World", available: true }
];

const resolvers = {
  Mutation: {
    updateBookAvailability: (_, { id, available }) => {
      const book = books.find(book => book.id === id);
      if (book) {
        book.available = available;
      }
      return book;
    }
  }
};

// Export schema and resolvers
// Your code here

Hint

Define a Query type with a books field returning a list of Book. Export the schema and resolvers for use in your GraphQL server.

Practice

1. What is the main purpose of a mutation in GraphQL?

easy

A. To modify data by adding, updating, or deleting it

B. To only fetch data without changes

C. To define the schema structure

D. To create a new query type

Solution

1. Step 1: Understand the role of mutations

   Mutations are designed to change data, unlike queries which only fetch data.

2. Step 2: Identify the correct purpose

   Since mutations add, update, or delete data, their main purpose is to modify data.

3. Final Answer:

   To modify data by adding, updating, or deleting it -> Option A

4. Quick Check:

   Mutation = Modify data [OK]

Hint: Mutations always change data, queries only read data [OK]

Common Mistakes:

• Confusing mutations with queries
• Thinking mutations only fetch data
• Believing mutations define schema only

2. Which of the following is the correct way to start a mutation operation in GraphQL?

easy

A. modify { updateUser(id: 1) { name } }

B. query { updateUser(id: 1) { name } }

C. update { updateUser(id: 1) { name } }

D. mutation { updateUser(id: 1) { name } }

Solution

1. Step 1: Identify the keyword for mutations

   GraphQL uses the keyword mutation to start mutation operations.

2. Step 2: Check the options

   Only mutation { updateUser(id: 1) { name } } uses the correct mutation keyword to start the operation.

3. Final Answer:

   mutation { updateUser(id: 1) { name } } -> Option D

4. Quick Check:

   Mutation keyword = mutation [OK]

Hint: Mutations always start with the 'mutation' keyword [OK]

Common Mistakes:

• Using 'query' instead of 'mutation'
• Using non-existent keywords like 'update' or 'modify'
• Omitting the mutation keyword

3. Given the mutation below, what will be the result if the user with id 2 exists?

mutation {
  updateUser(id: 2, input: {name: "Alice"}) {
    id
    name
  }
}

medium

A. Syntax error due to missing input type

B. { "data": { "updateUser": null } }

C. { "data": { "updateUser": { "id": 2, "name": "Alice" } } }

D. No change, returns old user data

Solution

1. Step 1: Understand mutation behavior

   The mutation updates user with id 2, changing the name to "Alice".

2. Step 2: Predict the returned data

   The mutation returns the updated user data with id and new name.

3. Final Answer:

   { "data": { "updateUser": { "id": 2, "name": "Alice" } } } -> Option C

4. Quick Check:

   Mutation updates and returns new data [OK]

Hint: Mutations return updated data if successful [OK]

Common Mistakes:

• Assuming mutation returns null on success
• Thinking mutation causes syntax error here
• Believing mutation returns old data without changes

4. Identify the error in this mutation:

mutation {
  addUser(name: "Bob")
  {
    id
    name
  }
}

medium

A. Mutation keyword is incorrect

B. Missing input argument object for addUser

C. Curly braces are misplaced

D. No error, mutation is correct

Solution

1. Step 1: Check mutation input format

   Mutations usually require input as an object, e.g., addUser(input: {name: "Bob"}).

2. Step 2: Identify missing input argument

   The mutation lacks the input argument object, causing an error.

3. Final Answer:

   Missing input argument object for addUser -> Option B

4. Quick Check:

   Input argument missing = error [OK]

Hint: Mutations need input objects for data changes [OK]

Common Mistakes:

• Forgetting to wrap inputs in an object
• Misplacing curly braces
• Confusing mutation keyword

5. You want to delete a user and then immediately fetch the updated user list in one mutation operation. Which approach correctly uses mutations to achieve this?

hard

A. Use a single mutation with two fields: deleteUser and users to delete and fetch updated list

B. Use a query to delete the user and a mutation to fetch the list

C. Use two separate mutations, one for deleting and one for fetching users

D. Use a mutation to delete the user and a query to fetch the updated list separately

Solution

1. Step 1: Understand mutation capabilities

   GraphQL mutations can include multiple fields to perform several operations in one request.

2. Step 2: Identify correct approach

   Using a single mutation with deleteUser and users fields allows deleting and fetching updated data atomically.

3. Final Answer:

   Use a single mutation with two fields: deleteUser and users to delete and fetch updated list -> Option A

4. Quick Check:

   Single mutation can do multiple related operations [OK]

Hint: Combine delete and fetch in one mutation for atomic update [OK]

Common Mistakes:

• Trying to delete with a query
• Splitting operations into separate requests unnecessarily
• Using mutation only for delete without fetching updated data