Input validation patterns in GraphQL

Introduction

Input validation helps make sure the data sent to a GraphQL server is correct and safe. It stops errors and bad data from causing problems.

When users submit forms with data like names, emails, or passwords.

When APIs receive data from other systems and need to check it before saving.

When you want to prevent invalid or harmful data from entering your database.

When you want to give clear error messages if the input is wrong.

When you want to enforce rules like minimum length or allowed characters.

Syntax

GraphQL

type Mutation {
  createUser(input: CreateUserInput!): User
}

input CreateUserInput {
  username: String!
  email: String!
  age: Int
}

# Validation happens in resolver or with custom scalars/directives

GraphQL schema defines the shape of input but does not do all validation automatically.

Use custom scalars or directives for advanced validation like patterns or ranges.

Examples

Defines required username and email, optional age.

GraphQL

input CreateUserInput {
  username: String!
  email: String!
  age: Int
}

Custom scalar Email can validate email format automatically.

GraphQL

scalar Email

input CreateUserInput {
  username: String!
  email: Email!
}

Directive enforces username length between 3 and 15 characters.

GraphQL

directive @length(min: Int, max: Int) on ARGUMENT_DEFINITION | INPUT_FIELD_DEFINITION

input CreateUserInput {
  username: String! @length(min: 3, max: 15)
}

Sample Program

This mutation creates a user after checking the input meets simple rules like username length and email format.

GraphQL

type Mutation {
  createUser(input: CreateUserInput!): User
}

input CreateUserInput {
  username: String!
  email: String!
  age: Int
}

type User {
  id: ID!
  username: String!
  email: String!
  age: Int
}

# Resolver example (pseudocode):
# if input.username.length < 3 then throw error
# if input.email does not match email pattern then throw error
# else create user and return

# Mutation example:
mutation {
  createUser(input: {username: "Sam", email: "[e-mail]", age: 25}) {
    id
    username
    email
    age
  }
}

Important Notes

GraphQL schema types ensure basic type safety but do not replace full validation.

Use custom scalars or validation libraries in resolvers for better input checks.

Clear error messages help users fix their input mistakes quickly.

Summary

Input validation keeps data safe and correct before saving or processing.

GraphQL schema defines input shapes but advanced checks need custom code or scalars.

Use directives, custom scalars, or resolver logic to enforce rules like length or format.

Practice

1. What is the main purpose of input validation in GraphQL schemas?

easy

A. To ensure data is safe and correct before processing

B. To speed up query execution

C. To automatically generate UI forms

D. To store data in multiple databases

Solution

1. Step 1: Understand input validation role

   Input validation checks data before it is saved or used to avoid errors or security issues.

2. Step 2: Identify main goal in GraphQL

   GraphQL uses input validation to keep data safe and correct before processing.

3. Final Answer:

   To ensure data is safe and correct before processing -> Option A

4. Quick Check:

   Input validation = data safety and correctness [OK]

Hint: Input validation means checking data before use [OK]

Common Mistakes:

• Confusing validation with performance optimization
• Thinking validation auto-generates UI
• Assuming validation stores data

2. Which of the following is a valid way to enforce input validation in a GraphQL schema?

easy

A. Using HTML tags in schema definitions

B. Writing SQL queries inside the schema

C. Adding CSS styles to input fields

D. Using custom scalar types for specific formats

Solution

1. Step 1: Review GraphQL schema capabilities

   GraphQL schemas can define custom scalar types to validate formats like email or date.

2. Step 2: Eliminate invalid options

   SQL queries, CSS, and HTML are unrelated to schema validation.

3. Final Answer:

   Using custom scalar types for specific formats -> Option D

4. Quick Check:

   Custom scalars = validation in schema [OK]

Hint: Custom scalars validate input formats in GraphQL [OK]

Common Mistakes:

• Confusing schema with UI styling
• Trying to embed SQL in schema
• Using HTML tags in schema definitions

3. Given this GraphQL input type and resolver snippet, what happens if the input 'username' is an empty string?

input UserInput {
  username: String!
}

resolver createUser(input: UserInput) {
  if (input.username.length === 0) {
    throw new Error('Username cannot be empty');
  }
  return saveUser(input);
}

medium

A. The resolver ignores the username field

B. The resolver throws an error and user is not saved

C. The user is saved with an empty username

D. The schema rejects the query before resolver runs

Solution

1. Step 1: Analyze resolver input check

   The resolver checks if username length is zero and throws an error if true.

2. Step 2: Understand effect of empty string input

   Empty string triggers the error, so user is not saved.

3. Final Answer:

   The resolver throws an error and user is not saved -> Option B

4. Quick Check:

   Empty username triggers error in resolver [OK]

Hint: Empty string triggers error in resolver check [OK]

Common Mistakes:

• Assuming schema rejects empty string automatically
• Thinking empty username is saved
• Ignoring resolver error handling

4. Identify the error in this GraphQL input validation snippet:

input ProductInput {
  price: Float!
}

resolver addProduct(input: ProductInput) {
  if (input.price < 0) {
    return 'Price must be positive';
  }
  saveProduct(input);
  return 'Product added';
}

medium

A. Returning a string error instead of throwing an error

B. Missing input type declaration

C. Using Float instead of Int for price

D. No validation for price being zero

Solution

1. Step 1: Check error handling in resolver

   The resolver returns a string on error instead of throwing an error, which may not stop execution properly.

2. Step 2: Understand proper error signaling

   Throwing an error is standard to halt processing and signal failure in GraphQL.

3. Final Answer:

   Returning a string error instead of throwing an error -> Option A

4. Quick Check:

   Errors should be thrown, not returned as strings [OK]

Hint: Throw errors, don't return error strings in resolvers [OK]

Common Mistakes:

• Returning error messages instead of throwing
• Confusing Float and Int types
• Ignoring zero price validation

5. You want to enforce that a user's email input is always lowercase and matches a valid email format in GraphQL. Which combination is the best approach?

hard

A. Use a directive to reject any uppercase letters without transformation

B. Only rely on GraphQL's String type without extra checks

C. Use a custom scalar for email format and transform input to lowercase in resolver

D. Validate email format in the database after saving

Solution

1. Step 1: Understand validation needs

   Email must be valid format and lowercase before saving or processing.

2. Step 2: Choose best GraphQL validation method

   Custom scalar can enforce format; resolver can transform input to lowercase.

3. Step 3: Evaluate other options

   Relying only on String misses validation; directives alone can't transform; database validation is late.

4. Final Answer:

   Use a custom scalar for email format and transform input to lowercase in resolver -> Option C

5. Quick Check:

   Custom scalar + resolver transform = best validation [OK]

Hint: Combine custom scalar and resolver for format and case [OK]

Common Mistakes:

• Skipping format validation
• Expecting directives to transform input
• Validating only after saving data