Interface types in GraphQL

Introduction

Interface types let you define a common set of fields that different data types share. This helps organize data and makes queries simpler.

When you have different data types that share some common fields, like different kinds of users.

When you want to write queries that work on multiple types without repeating code.

When you want to ensure certain fields always exist on several types.

When you want to build flexible APIs that can return different types but with shared fields.

Syntax

GraphQL

interface InterfaceName {
  field1: Type1
  field2: Type2
}

type TypeA implements InterfaceName {
  field1: Type1
  field2: Type2
  extraField: ExtraType
}

type TypeB implements InterfaceName {
  field1: Type1
  field2: Type2
  anotherField: AnotherType
}

Interfaces define fields without specifying how they work.

Types that implement the interface must include all its fields.

Examples

This example shows an interface Animal with two fields. Both Dog and Cat types implement it and add their own fields.

GraphQL

interface Animal {
  name: String
  age: Int
}

type Dog implements Animal {
  name: String
  age: Int
  breed: String
}

type Cat implements Animal {
  name: String
  age: Int
  color: String
}

Here, Vehicle interface is shared by Car and Bike. Each type adds unique fields.

GraphQL

interface Vehicle {
  make: String
  model: String
}

type Car implements Vehicle {
  make: String
  model: String
  doors: Int
}

type Bike implements Vehicle {
  make: String
  model: String
  type: String
}

Sample Program

This example defines a Person interface with id and name. Both Employee and Customer implement it. The query fetches all people showing only the shared fields.

GraphQL

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

type Employee implements Person {
  id: ID!
  name: String!
  salary: Float
}

type Customer implements Person {
  id: ID!
  name: String!
  purchaseCount: Int
}

# Query to get all persons with their common fields
query {
  people {
    id
    name
  }
}

Important Notes

Interfaces help keep your API organized and consistent.

You can use fragments in queries to get fields from specific types implementing an interface.

Summary

Interfaces define shared fields for multiple types.

Types must implement all interface fields.

Use interfaces to write flexible and reusable queries.

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