Default resolvers in GraphQL - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a default resolver in GraphQL?

A default resolver is the built-in function GraphQL uses to fetch the value of a field when no custom resolver is provided. It simply returns the property of the source object that matches the field name.

beginner

How does GraphQL's default resolver find the value for a field?

It looks for a property on the parent object with the same name as the field and returns its value directly.

intermediate

When would you need to write a custom resolver instead of relying on the default resolver?

When the data needs transformation, fetching from another source, or combining multiple fields, you write a custom resolver. The default resolver only returns existing properties as-is.

beginner

True or False: The default resolver can handle nested objects automatically.

True. If the field's value is an object, the default resolver returns it, and GraphQL continues resolving fields on that nested object.

beginner

What happens if the default resolver does not find a matching property on the source object?

It returns null or undefined, which means the field will have no value in the query result unless a custom resolver handles it.

What does the default resolver in GraphQL do when resolving a field?

A Returns the property of the source object matching the field name

B Always returns null

C Fetches data from a database automatically

D Transforms the data before returning

The default resolver returns the property on the source object that matches the field name.

When is a custom resolver necessary instead of using the default resolver?

A When data needs to be fetched from another source or transformed

B When the field name matches the source object property

C When the query is simple

D When the default resolver returns the correct value

Custom resolvers are needed when data requires fetching from other sources or transformation.

If a field's value is an object, how does the default resolver handle it?

A It throws an error

B It returns null

C It returns the object and GraphQL resolves nested fields automatically

D It converts the object to a string

The default resolver returns the object, allowing GraphQL to resolve nested fields.

What will the default resolver return if the source object does not have the requested field?

A An error message

B Null or undefined

C The entire source object

D A default string value

If the property is missing, the default resolver returns null or undefined.

Which of the following is NOT true about default resolvers?

A They handle nested objects by returning them

B They return the property matching the field name

C They are used when no custom resolver is defined

D They automatically fetch data from databases

Default resolvers do not fetch data from databases automatically; they just return existing properties.

Explain how GraphQL's default resolver works when resolving a field in a query.

Think about how GraphQL finds the value without custom code.

Describe a situation where you would need to write a custom resolver instead of relying on the default resolver.

When the data is not directly available as a simple property.

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