Abstract type resolution in GraphQL - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is abstract type resolution in GraphQL?

It is the process of determining the concrete object type that implements an abstract type like an interface or union during query execution.

beginner

Name two GraphQL abstract types that require resolution.

Interfaces and unions are the two abstract types that need resolution to find the actual object type at runtime.

intermediate

How does GraphQL resolve an abstract type?

GraphQL calls a resolveType function or uses a default mechanism to identify the concrete type of the object returned.

intermediate

Why is abstract type resolution important in GraphQL?

It allows clients to query fields specific to the concrete type while using a common interface or union, enabling flexible and type-safe queries.

advanced

What happens if GraphQL cannot resolve the concrete type of an abstract type?

The query will fail with an error because GraphQL needs to know the exact type to return the correct fields.

Which GraphQL types require abstract type resolution?

A Directive and Scalar

B Scalar and Enum

C Object and Input

D Interface and Union

Only Interface and Union types are abstract and require resolution to a concrete object type.

What function is commonly used to resolve an abstract type in GraphQL?

A resolveObject

B resolveType

C resolveQuery

D resolveField

The resolveType function is used to determine the concrete type of an abstract type.

If a GraphQL query requests fields from an interface, what must happen during execution?

A The interface is ignored and fields are returned as null

B The query is converted to a union type

C The concrete type implementing the interface must be identified

D The query fails immediately

GraphQL must identify the concrete type to return the correct fields for the interface.

What is the result if abstract type resolution fails in GraphQL?

A The query returns an error

B The query returns empty data

C The query returns default scalar values

D The query returns all possible fields

Failure to resolve the concrete type causes the query to error out.

Which of the following is NOT an abstract type in GraphQL?

A Object

B Interface

C Union

D Abstract

Object types are concrete, not abstract. Interface and Union are abstract types.

Explain how GraphQL resolves an abstract type during query execution.

Think about how GraphQL knows which actual type to use when you query an interface or union.

Why is abstract type resolution necessary in GraphQL schemas?

Consider how clients can query different types through a common abstract type.

Practice

1. What is the main purpose of the __resolveType function in GraphQL when using interfaces or unions?

easy

A. To fetch data from the database

B. To validate the query syntax before execution

C. To define new scalar types

D. To determine the specific object type to return for an abstract type

Solution

1. Step 1: Understand abstract types in GraphQL

   Abstract types like interfaces or unions can represent multiple object types.

2. Step 2: Role of __resolveType

   This function tells GraphQL which concrete type to use for the returned data.

3. Final Answer:

   To determine the specific object type to return for an abstract type -> Option D

4. Quick Check:

   Abstract type resolution = determine specific type [OK]

Hint: Remember: __resolveType picks the exact type [OK]

Common Mistakes:

• Confusing __resolveType with data fetching
• Thinking it validates query syntax
• Assuming it defines scalar types

2. Which of the following is the correct way to define a __resolveType function in a GraphQL resolver object?

easy

A. resolveType(obj) { return obj.typeName; }

B. __resolveType(obj) { return obj.kind; }

C. __resolveType(obj) { return obj.__typename; }

D. __resolveType(obj) { return obj.type; }

Solution

1. Step 1: Check the naming and return value

   The function must be named exactly __resolveType and return a string matching a type name.

2. Step 2: Match the returned value to the data field

   Commonly, the field __typename holds the type name, so returning obj.__typename is correct.

3. Final Answer:

   __resolveType(obj) { return obj.__typename; } -> Option C

4. Quick Check:

   Correct function name and return field = __resolveType(obj) { return obj.__typename; } [OK]

Hint: Function must be named exactly __resolveType [OK]

Common Mistakes:

• Using wrong function name like resolveType
• Returning incorrect property like type
• Returning undefined or wrong field

3. Given this resolver snippet for a union type:

__resolveType(obj) {
  if (obj.price) return 'Book';
  if (obj.author) return 'Author';
  return null;
}

What will be the resolved type for { price: 20, author: 'John' }?

medium

A. Book

B. Author

C. null

D. Error

Solution

1. Step 1: Check conditions in order

   The function first checks if obj.price exists, which is true here.

2. Step 2: Return first matching type

   Since obj.price is true, it returns 'Book' immediately without checking further.

3. Final Answer:

   Book -> Option A

4. Quick Check:

   First true condition returns 'Book' [OK]

Hint: Check conditions top to bottom, first match wins [OK]

Common Mistakes:

• Assuming it returns 'Author' because author field exists
• Thinking it returns null if multiple fields exist
• Expecting an error for multiple matches

4. You wrote this __resolveType function:

__resolveType(obj) {
  if (obj.kind === 'User') return 'User';
  if (obj.kind === 'Admin') return 'Admin';
}

But your GraphQL query returns null for the type. What is the likely problem?

medium

A. Function name should be resolveType without underscores

B. Missing a return statement for unmatched cases

C. The kind field does not exist in obj

D. GraphQL does not support __resolveType for interfaces

Solution

1. Step 1: Check function completeness

   The function lacks a return for cases when obj.kind is neither 'User' nor 'Admin'.

2. Step 2: Understand GraphQL behavior

   If no type is returned, GraphQL resolves the type as null, causing query issues.

3. Final Answer:

   Missing a return statement for unmatched cases -> Option B

4. Quick Check:

   Always return a type or null explicitly [OK]

Hint: Always return a type or null in __resolveType [OK]

Common Mistakes:

• Using wrong function name without underscores
• Assuming missing return defaults to a type
• Believing GraphQL doesn't support __resolveType

5. You have a GraphQL interface Vehicle implemented by types Car and Bike. Your __resolveType function is:

__resolveType(obj) {
  return obj.wheels === 4 ? 'Car' : 'Bike';
}

If an object has { wheels: 0 }, what will happen when querying this interface?

hard

A. It will resolve to 'Bike' because wheels is not 4

B. It will resolve to 'Car' because 0 is falsy

C. It will cause a runtime error due to invalid wheels

D. It will return null and cause query failure

Solution

1. Step 1: Evaluate the ternary condition

   The condition checks if obj.wheels === 4. For 0, this is false.

2. Step 2: Determine returned type

   Since condition is false, it returns 'Bike'.

3. Final Answer:

   It will resolve to 'Bike' because wheels is not 4 -> Option A

4. Quick Check:

   Condition false returns 'Bike' [OK]

Hint: Check exact equality, not truthiness, in __resolveType [OK]

Common Mistakes:

• Confusing falsy 0 with true condition
• Expecting runtime error for zero wheels
• Assuming null return causes failure