Interface types in GraphQL - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is an interface type in GraphQL?

An interface type in GraphQL is a way to define a set of fields that multiple object types can share. It acts like a contract that those object types must follow.

beginner

How do you declare an interface type in GraphQL schema?

You use the interface keyword followed by the interface name and the fields it contains. For example:

interface Character {
  id: ID!
  name: String!
}

beginner

How does an object type implement an interface in GraphQL?

An object type implements an interface by using the implements keyword and including all fields defined in the interface. For example:

type Human implements Character {
  id: ID!
  name: String!
  homePlanet: String
}

intermediate

Why use interface types in GraphQL?

Interfaces help to reuse common fields across different object types and allow queries to be more flexible by returning different types that share the same interface.

intermediate

Can you query an interface type directly in GraphQL?

Yes, you can query an interface type, but you must specify which fields you want from the interface and also use inline fragments to get fields specific to the implementing object types.

What keyword is used to define an interface type in GraphQL?

A interface

B type

C implements

D union

The interface keyword is used to define an interface type in GraphQL.

If an object type implements an interface, what must it do?

A Ignore the interface fields

B Only include some fields from the interface

C Include all fields defined in the interface

D Rename the interface fields

An object type must include all fields defined in the interface it implements.

Which of the following is true about querying interfaces?

A Interfaces are only used for mutations

B You cannot query interfaces directly

C You can query an interface and get fields from all implementing types without specifying them

D You can query an interface but need inline fragments to get fields from specific types

When querying an interface, you specify interface fields and use inline fragments to get fields from specific implementing types.

What is a main benefit of using interface types in GraphQL?

A They allow reuse of common fields across types

B They speed up database queries

C They replace unions

D They prevent any type from having unique fields

Interfaces allow reuse of common fields across different object types.

Which keyword does an object type use to show it follows an interface?

A inherits

B implements

C extends

D uses

The implements keyword is used by object types to indicate they follow an interface.

Explain what an interface type is in GraphQL and why it is useful.

Think about how different characters in a story might share common traits.

Describe how you would query data from an interface type and its implementing object types.

Remember you need to ask for common fields and then specify extra fields for each type.

Practice

1. What is the main purpose of an interface in GraphQL?

easy

A. To define a set of fields that multiple types must implement

B. To create a new scalar type

C. To specify a query operation

D. To define a mutation resolver

Solution

1. Step 1: Understand the role of interfaces

   Interfaces in GraphQL define common fields that multiple types share.

2. Step 2: Compare options with interface purpose

   Only To define a set of fields that multiple types must implement correctly states that interfaces define shared fields for multiple types.

3. Final Answer:

   To define a set of fields that multiple types must implement -> Option A

4. Quick Check:

   Interface = shared fields [OK]

Hint: Interfaces define shared fields for multiple types [OK]

Common Mistakes:

• Confusing interfaces with scalar types
• Thinking interfaces define queries or mutations
• Assuming interfaces can be instantiated directly

2. Which of the following is the correct syntax to declare an interface named Node with a field id of type ID!?

easy

A. interface Node { id: ID! }

B. interface Node { id: ID }

C. interface Node id: ID!

D. interface Node { id: String! }

Solution

1. Step 1: Recall GraphQL interface syntax

   Interfaces are declared with the keyword interface, followed by the name and fields with types.

2. Step 2: Check field type correctness

   The field id must be non-nullable ID!, so interface Node { id: ID! } matches exactly.

3. Final Answer:

   interface Node { id: ID! } -> Option A

4. Quick Check:

   Correct interface syntax = interface Node { id: ID! } [OK]

Hint: Use 'interface Name { field: Type! }' syntax exactly [OK]

Common Mistakes:

• Omitting the exclamation mark for non-nullable
• Using wrong scalar type like String instead of ID
• Missing braces around fields

3. Given the interface and types below, what will the query { search { id name } return?

interface SearchResult {
  id: ID!
}
type User implements SearchResult {
  id: ID!
  name: String!
}
type Product implements SearchResult {
  id: ID!
  name: String!
  price: Float!
}

medium

A. A list of objects each with fields id and name

B. A list of objects with only id fields

C. An error because price is missing in the interface

D. A syntax error because name is not in the interface

Solution

1. Step 1: Review the interface definition

   The SearchResult interface only defines id: ID!. The name field is present in implementing types but not in the interface.

2. Step 2: Analyze the query against the interface

   Querying name directly on search (SearchResult) fails because it is not defined on the interface. GraphQL requires inline fragments for type-specific fields like ... on User { name }.

3. Final Answer:

   A syntax error because name is not in the interface -> Option D

4. Quick Check:

   Directly query only interface fields; use fragments for type-specific [OK]

Hint: Query only interface fields directly; use fragments for type-specific fields [OK]

Common Mistakes:

• Assuming only interface fields can be queried
• Expecting error if extra fields exist in types
• Confusing interface fields with type-specific fields

4. Consider the following schema snippet:

interface Vehicle {
  id: ID!
  speed: Int!
}
type Car implements Vehicle {
  id: ID!
  speed: Int!
  brand: String!
}
type Bike implements Vehicle {
  id: ID!
  brand: String!
}

What is the error in this schema?

medium

A. Car type has an extra field brand not in Vehicle interface

B. Bike type is missing the required speed field from Vehicle interface

C. Vehicle interface cannot have fields of type Int

D. Interface Vehicle must be a type, not an interface

Solution

1. Step 1: Check interface field requirements

   All types implementing an interface must have all interface fields with matching types.

2. Step 2: Verify Bike type fields

   Bike implements Vehicle but lacks the speed field required by Vehicle, causing an error.

3. Final Answer:

   Bike type is missing the required speed field from Vehicle interface -> Option B

4. Quick Check:

   Implementing types must have all interface fields [OK]

Hint: Check all interface fields are implemented in each type [OK]

Common Mistakes:

• Thinking extra fields in types cause errors
• Ignoring missing interface fields in types
• Confusing interface with type declaration rules

5. You want to design a GraphQL schema where multiple types like Book and Movie share fields id and title, but each has unique fields too. How should you use interfaces to achieve this?

hard

A. Use scalar types for id and title in each type separately

B. Create a union type of Book and Movie without shared fields

C. Define an interface Item with id and title, then have Book and Movie implement Item and add their unique fields

D. Define Book and Movie as separate types without interfaces

Solution

1. Step 1: Identify shared fields and unique fields

   Both Book and Movie share id and title, but have unique fields.

2. Step 2: Use interface for shared fields

   Defining an interface Item with shared fields and implementing it in Book and Movie allows reuse and flexibility.

3. Step 3: Add unique fields in each type

   Book and Movie can add their own fields beyond the interface.

4. Final Answer:

   Define an interface Item with id and title, then have Book and Movie implement Item and add their unique fields -> Option C

5. Quick Check:

   Interfaces = shared fields + types add unique fields [OK]

Hint: Use interfaces for shared fields, types add unique fields [OK]

Common Mistakes:

• Using union types instead of interfaces for shared fields
• Duplicating shared fields in each type without interface
• Misusing scalar types for shared fields