Graphql-merge Typegraphql

5 min read Oct 01, 2024

Combining Power: A Guide to Merging Types in TypeGraphQL with GraphQL

GraphQL, the query language for APIs, has gained immense popularity for its flexibility and efficiency. When building complex GraphQL applications, organizing your schema becomes crucial. TypeGraphQL, a powerful library for building GraphQL servers with TypeScript, offers a streamlined approach to type definition. But how do you handle scenarios where you need to combine types from multiple sources? This is where the concept of type merging comes into play.

Why Merge Types?

Merging types in TypeGraphQL provides several advantages, particularly when working on large-scale applications:

Modularization: Divide your schema into smaller, manageable units. Each module can contain its own set of types, making code easier to maintain and understand.
Reusability: Define common types that can be shared across multiple modules, reducing redundancy and improving consistency.
Flexibility: Integrate third-party libraries or custom logic that may introduce new types without affecting your existing schema.

How to Merge Types in TypeGraphQL

TypeGraphQL offers a straightforward approach to merging types using the @ObjectType and @Field decorators. Here's a breakdown:

Define Base Types: Create separate type definitions for each module or component.

// Module A
@ObjectType()
export class User {
    @Field()
    id: string;

    @Field()
    name: string;
}

// Module B
@ObjectType()
export class Product {
    @Field()
    id: string;

    @Field()
    name: string;

    @Field()
    price: number;
}

Merge Types: Create a new type that extends the base types using the @ObjectType() decorator.

// Merged Type
@ObjectType({ implements: [User, Product] })
export class UserProduct implements User, Product {
    @Field(() => String)
    id: string;

    @Field(() => String)
    name: string;

    @Field(() => Number)
    price?: number; // Optional field from Product type
}

Utilize the Merged Type: Use the newly created UserProduct type in your resolvers and queries.

@Resolver()
export class UserProductResolver {
    @Query(() => UserProduct)
    async getUserProduct() {
        // Implement your data fetching logic
    }
}

Practical Example: Extending User Data with Address Information

Imagine you have a basic User type and want to add an Address type to provide complete user information.

Base Types:

@ObjectType()
export class User {
    @Field()
    id: string;

    @Field()
    name: string;
}

@ObjectType()
export class Address {
    @Field()
    street: string;

    @Field()
    city: string;
}

Merged Type:

@ObjectType({ implements: [User, Address] })
export class FullUser implements User, Address {
    @Field(() => String)
    id: string;

    @Field(() => String)
    name: string;

    @Field(() => String)
    street: string;

    @Field(() => String)
    city: string;
}

Resolver:

@Resolver()
export class FullUserResolver {
    @Query(() => FullUser)
    async getFullUser() {
        // Logic to retrieve user data and address
    }
}

Tips and Best Practices

Organize Modules: Structure your types based on logical modules or domains for better organization and maintainability.
Use Interfaces: Define interfaces for common properties across types to enforce consistency and improve type safety.
Avoid Redundancy: Merged types should aim to reduce redundancy by inheriting properties from existing types.
Document Clearly: Clearly document your merged types and their relationship to the base types for improved understanding.

Conclusion

Type merging in TypeGraphQL provides a powerful way to structure your GraphQL schema and build scalable applications. By understanding the concept and following best practices, you can create a robust and flexible GraphQL API using the power of TypeScript.