Default resolvers in GraphQL

Introduction

Default resolvers help GraphQL find and return data automatically without extra code.

When you want GraphQL to fetch simple fields from your data without writing custom code.

When your data structure matches your GraphQL schema fields directly.

When you want to quickly set up a GraphQL API that returns data from objects or databases.

When you want to avoid writing resolver functions for every field in your schema.

Syntax

GraphQL

No special code needed. GraphQL uses default resolvers automatically for fields that match your data.

Default resolvers work by matching field names in your schema to keys in your data objects.

If a field needs special handling, you can write a custom resolver to override the default.

Examples

If your data for user is { id: "1", name: "Anna" }, GraphQL returns these fields automatically.

GraphQL

type User {
  id: ID
  name: String
}

type Query {
  user: User
}

Default resolvers fetch id and name from the user object by matching field names.

GraphQL

const user = { id: "1", name: "Anna" };

// GraphQL default resolver returns user.id and user.name without extra code

Sample Program

This GraphQL schema defines a User type and a user query. The resolver returns userData. Default resolvers automatically return id and name from userData.

GraphQL

type User {
  id: ID
  name: String
}

type Query {
  user: User
}

const userData = { id: "1", name: "Anna" };

const resolvers = {
  Query: {
    user: () => userData
  }
};

Important Notes

Default resolvers only work when your data keys match your schema fields exactly.

If a field is a function or needs transformation, default resolvers won't work and you must write custom resolvers.

Summary

Default resolvers automatically return data by matching schema fields to data keys.

They save time by avoiding writing simple resolver functions.

Use custom resolvers when you need special data processing.

Practice

1. What is the main purpose of default resolvers in GraphQL?

easy

A. To write custom logic for every field in the schema

B. To automatically return data by matching schema fields to data keys

C. To validate user input before querying the database

D. To handle errors during query execution

Solution

1. Step 1: Understand default resolver behavior

   Default resolvers automatically map schema fields to matching data keys without extra code.

2. Step 2: Compare with other options

   Options B, C, and D describe custom logic, validation, and error handling, which are not default resolver roles.

3. Final Answer:

   To automatically return data by matching schema fields to data keys -> Option B

4. Quick Check:

   Default resolver = automatic field-to-key mapping [OK]

Hint: Default resolvers match schema fields to data keys automatically [OK]

Common Mistakes:

• Thinking default resolvers handle validation
• Confusing default resolvers with custom resolvers
• Assuming default resolvers manage errors

2. Which of the following is the correct way to define a GraphQL schema field that uses a default resolver?

easy

A. type Query { user: User() }

B. type Query { user: User @customResolver }

C. type Query { user: User }

D. type Query { user: User -> resolver }

Solution

1. Step 1: Identify correct schema syntax

   GraphQL schema fields are defined as fieldName: Type without extra syntax for default resolvers.

2. Step 2: Analyze options

   type Query { user: User } uses correct syntax. Options B, C, and D include invalid or non-standard syntax.

3. Final Answer:

   type Query { user: User } -> Option C

4. Quick Check:

   Default resolver needs simple field: Type syntax [OK]

Hint: Default resolvers need simple field: Type syntax in schema [OK]

Common Mistakes:

• Adding parentheses after type name
• Using custom directives without definition
• Using arrow or other symbols in schema

3. Given the schema type Query { book: Book } and data { book: { title: "1984", author: "Orwell" } }, what will the default resolver return for the field book?

medium

A. { title: "1984", author: "Orwell" }

B. "1984"

C. null

D. Error: Resolver not found

Solution

1. Step 1: Understand default resolver behavior

   The default resolver returns the data object matching the field name if it exists.

2. Step 2: Match field to data

   The field book matches the data key book which contains the object with title and author.

3. Final Answer:

   { title: "1984", author: "Orwell" } -> Option A

4. Quick Check:

   Field book returns matching data object [OK]

Hint: Default resolver returns matching data object for field name [OK]

Common Mistakes:

• Returning only one property instead of whole object
• Expecting null if no custom resolver is defined
• Assuming error if resolver is not explicitly written

4. You wrote a GraphQL schema with type Query { user: User } but your query returns null for user. What is a likely cause related to default resolvers?

medium

A. The data source does not have a matching key named 'user'

B. You forgot to write a custom resolver for 'user'

C. The schema syntax is invalid

D. GraphQL does not support default resolvers

Solution

1. Step 1: Recall default resolver behavior

   Default resolvers return data matching the field name from the data source.

2. Step 2: Identify cause of null result

   If the data source lacks a key named 'user', the default resolver returns null.

3. Final Answer:

   The data source does not have a matching key named 'user' -> Option A

4. Quick Check:

   Missing data key causes null from default resolver [OK]

Hint: Check data keys match schema fields for default resolver to work [OK]

Common Mistakes:

• Assuming custom resolver is always required
• Blaming schema syntax without checking data
• Thinking default resolvers are unsupported

5. You have a schema type Query { product: Product } and data { product: { id: 1, name: "Pen", price: 2.5 } }. You want to return the price with a 10% discount but keep default resolvers for other fields. How should you implement this?

hard

A. Use default resolvers only; custom resolvers cannot be mixed

B. Write a custom resolver for the entire 'product' field in Query

C. Modify the data source to store discounted price

D. Write a custom resolver only for the 'price' field inside Product type

Solution

1. Step 1: Understand resolver layering

   Default resolvers work per field; you can override specific fields with custom resolvers.

2. Step 2: Apply custom resolver to 'price' only

   Write a custom resolver for 'price' to apply discount, leaving other fields to default resolvers.

3. Step 3: Avoid overriding entire 'product' resolver

   Overriding entire 'product' would require manually returning all fields, losing default resolver benefits.

4. Final Answer:

   Write a custom resolver only for the 'price' field inside Product type -> Option D

5. Quick Check:

   Custom resolver per field + default resolvers for others [OK]

Hint: Override only needed fields with custom resolvers, keep defaults for rest [OK]

Common Mistakes:

• Overriding entire parent field unnecessarily
• Trying to mix default and custom resolvers at root level only
• Modifying data source instead of resolver logic